Update Fenix docs (#12250)

Co-authored-by: Mathieu Pillard <diox@users.noreply.github.com>

docs/fenix.md

@@ -2,42 +2,48 @@
 
 It is sometimes useful to test AMO with a Fenix build pointing to -dev or -stage given the tight integration between these two components. We describe how to construct a custom Fenix build below.
 
-The first step is to get a working Android development environment. The easiest way is to install [Android Studio](https://developer.android.com/studio). Follow the [Fenix build instructions](https://github.com/mozilla-mobile/fenix#build-instructions) to get the Fenix code and construct a debug build (which will download the dependencies and compile the code).
+The first step is to get a working Android development environment. The easiest way is to install [Android Studio](https://developer.android.com/studio). Follow the [Fenix build instructions](https://github.com/mozilla-mobile/firefox-android/tree/main/fenix) to get the Fenix code and construct a debug build (which will download the dependencies and compile the code).
 
-Android Studio provides virtual devices _via_ the [AVD Manager](https://developer.android.com/studio/run/managing-avds) in case you do not have a real Android-compatible device. In order to run the Fenix debug build created before, either configure a new virtual device or use your own device, then run the following command in the Fenix root directory:
+Android Studio provides virtual devices _via_ the [AVD Manager](https://developer.android.com/studio/run/managing-avds) in case you do not have a real Android-compatible device. In order to run the Fenix debug build created before, either configure a new virtual device or use your own device, then run the following command in the Fenix directory (inside the `firefox-android` repository):
 
 ```
-$ ./gradlew installDebug
+$ cd /path/to/firefox-android/fenix/
+$ ./gradlew installFenixDebug
 ```
 
 For more information, you can find more information on this page: https://developer.android.com/studio/build/building-cmdline#RunningOnDevice
 
 ## Custom build for AMO
 
-Once you ensured that you can compile, install and run a debug Fenix build, let's update the build configuration for AMO -stage. Only a few values have to be changed in the `app/build.gradle` file as shown below:
+Once you ensured that you can compile, install and run a debug Fenix build, let's update the build configuration for AMO -stage. Only a few values have to be changed in the `/path/to/firefox-android/fenix/app/build.gradle` file as shown below:
 
 ```diff
-
-diff --git a/app/build.gradle b/app/build.gradle
-index cd7c44f2d..78e0ffd2e 100644
---- a/app/build.gradle
-+++ b/app/build.gradle
-@@ -38,11 +38,11 @@ android {
-         resValue "bool", "IS_DEBUG", "false"
+diff --git a/fenix/app/build.gradle b/fenix/app/build.gradle
+index 81d60db1fc..f3c4d53ebd 100644
+--- a/fenix/app/build.gradle
++++ b/fenix/app/build.gradle
+@@ -45,9 +45,9 @@ android {
          buildConfigField "boolean", "USE_RELEASE_VERSIONING", "false"
+         buildConfigField "String", "GIT_HASH", "\"\"" // see override in release builds for why it's blank.
          // This should be the "public" base URL of AMO.
 -        buildConfigField "String", "AMO_BASE_URL", "\"https://addons.mozilla.org\""
--        buildConfigField "String", "AMO_COLLECTION_NAME", "\"7dfae8669acc4312a65e8ba5553036\""
+-        buildConfigField "String", "AMO_COLLECTION_NAME", "\"Extensions-for-Android\""
 -        buildConfigField "String", "AMO_COLLECTION_USER", "\"mozilla\""
 +        buildConfigField "String", "AMO_BASE_URL", "\"https://addons.allizom.org\""
 +        buildConfigField "String", "AMO_COLLECTION_NAME", "\"fenix\""
 +        buildConfigField "String", "AMO_COLLECTION_USER", "\"11686491\""
+         // These add-ons should be excluded for Mozilla Online builds.
+         buildConfigField "String[]", "MOZILLA_ONLINE_ADDON_EXCLUSIONS",
+                 "{" +
+@@ -62,7 +62,7 @@ android {
+                         "\"adnauseam@rednoise.org\"" +
+                 "}"
          // This should be the base URL used to call the AMO API.
 -        buildConfigField "String", "AMO_SERVER_URL", "\"https://services.addons.mozilla.org\""
-+        buildConfigField "String", "AMO_SERVER_URL", "\"https://addons.allizom.org\""
++        buildConfigField "String", "AMO_SERVER_URL", "\"https://services.addons.allizom.org\""
+
          def deepLinkSchemeValue = "fenix-dev"
          buildConfigField "String", "DEEP_LINK_SCHEME", "\"$deepLinkSchemeValue\""
-         manifestPlaceholders = [
 ```
 
 You may want to use your own values for `AMO_COLLECTION_USER` and `AMO_COLLECTION_NAME`. These values are used to determine which add-ons can be installed in Fenix based on an AMO collection. In this diff above, the ["fenix" collection](https://addons.allizom.org/en-US/firefox/collections/11686491/fenix/) is owned by [willdurand](https://github.com/willdurand).
@@ -45,7 +51,8 @@ You may want to use your own values for `AMO_COLLECTION_USER` and `AMO_COLLECTIO
 Recompile and install the custom build with the following command:
 
 ```
-$ ./gradlew installDebug
+$ cd /path/to/firefox-android/fenix/
+$ ./gradlew installFenixDebug
 ```
 
 The last step is to configure some prefs to be able to install -dev/-stage add-ons. Open `about:config` in your Fenix custom build and create/update the following prefs:
@@ -55,4 +62,4 @@ The last step is to configure some prefs to be able to install -dev/-stage add-o
 
 You should now be able to install add-ons from https://addons.allizom.org/ based on the list specified above.
 
-Note: the commands above usually output different APKs for different architectures. All generated APKs can be found in `app/build/outputs/apk/debug` in case you need to share them.
+Note: the commands above usually output different APKs for different architectures. All generated APKs can be found in `fenix/app/build/outputs/apk/fenix/debug` in case you need to share them.