Bug 1823842 - Explicitly only build the wrench library on android. r=gfx-reviewers,jnicol

When upgrading the NDK to r23, the wrench builds for android fail
because cargo apk starts adding flags to the cargo rustc call it does,
and that's not compatible with the wrench crate having both a lib and a
bin target.

As cargo apk only packages the lib in the apk, we can just be explicit
and build the library only.

Differential Revision: https://phabricator.services.mozilla.com/D173244

gfx/wr/wrench/android.txt

@@ -25,7 +25,7 @@ Compiling and running:
         cd wrench
         export ANDROID_SDK_ROOT=$HOME/.mozbuild/android-sdk-linux # exact path may vary
         export ANDROID_NDK_ROOT=$HOME/.mozbuild/android-ndk-r21d  # exact path may vary
-        cargo apk build
+        cargo apk build --lib
 
     Install the APK:
         adb install -r ../target/debug/apk/wrench.apk
@@ -57,7 +57,7 @@ Release mode:
         cd wrench
         export ANDROID_SDK_ROOT=$HOME/.mozbuild/android-sdk-linux # exact path may vary
         export ANDROID_NDK_ROOT=$HOME/.mozbuild/android-ndk-r21d  # exact path may vary
-        cargo apk build --release
+        cargo apk build --lib --release
 
     Now the APK at ../target/release/apk/wrench.apk
     should be signed and installable (you may need to uninstall the debug APK first if you

taskcluster/scripts/misc/wrench-android-build.sh

@@ -16,9 +16,9 @@ export ANDROID_SDK_ROOT="${MOZ_FETCHES_DIR}/android-sdk-linux"
 export ANDROID_NDK_ROOT="${MOZ_FETCHES_DIR}/android-ndk"
 
 if [ "$MODE" == "debug" ]; then
-    ../cargo-apk/bin/cargo-apk apk build --frozen --verbose
+    ../cargo-apk/bin/cargo-apk apk build --frozen --verbose --lib
 elif [ "$MODE" == "release" ]; then
-    ../cargo-apk/bin/cargo-apk apk build --frozen --verbose --release
+    ../cargo-apk/bin/cargo-apk apk build --frozen --verbose --lib --release
 else
     echo "Unknown mode '${MODE}'; must be 'debug' or 'release'"
     exit 1