Android: Set Firebase app IDs at runtime.

Currently, the ID is part of BuildConfig.java, which is generated at
build time. This doesn't work for Trichrome, since WebView and Chrome
have different Firebase app IDs.

This CL moves the IDs to code and then sets the correct ID at
startup.

Can't remove firebase_app_id from build files until the downstream
uses are gone.

Bug: 957618
Change-Id: I7b7d787eaf3e5cd00452bdf3696e93751304a6a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592473
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#656838}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 753d2d8b9c1277c18cde94aa400c1a8ef5f642fc

config/android/rules.gni

@@ -1943,6 +1943,10 @@ if (enable_java_templates) {
         defines += [ "_IS_UBSAN" ]
       }
 
+      if (is_chrome_branded) {
+        defines += [ "_IS_CHROME_BRANDED" ]
+      }
+
       if (invoker.use_final_fields) {
         forward_variables_from(invoker,
                                [