fix: shrink the android release with r8

Release builds had R8 and resource shrinking off, so every class of
every dependency went into the APK unminified: the published 0.1.62 is
42.8 MB for arm64, 25.85 MB of it dex (PF9 in the audit).

Turn both on through the properties build.gradle already reads
(`android.enableMinifyInReleaseBuilds` and
`android.enableShrinkResourcesInReleaseBuilds`). The consumer rules of
React Native, Expo, Firebase, ML Kit and the other libraries cover what
they load by reflection. The app's own package is kept whole, which
costs a few KB and keeps its frames readable in an obfuscated stack
trace.

Measured on a local x86_64 release build:

- APK: 44.7 MB -> 38.0 MB (download size 23.8 MB -> 17.2 MB)
- dex: 30.4 MB in 4 files -> 11.3 MB in 2 files
- classes: 27,631 -> 16,724

With `-dontobfuscate` the APK would be 38.8 MB (13.5 MB of dex), so
obfuscation stays on. Library frames in a release stack trace are now
obfuscated; retrace them with that build's
`android/app/build/outputs/mapping/release/mapping.txt`.

Checked on the emulator: cold start in light and dark mode, German and
Russian strings, SENDTO and VIEW mailto and SEND share intents, every
sign-in screen, the QR scanner with ML Kit, and an FCM data message
starting the headless push task.

android/app/proguard-rules.pro

@@ -12,3 +12,12 @@
 -keep class com.facebook.react.turbomodule.** { *; }
 
 # Add any project specific keep options here:
+
+# The app's own native code (com.anonymous.bulwarkmobile). What R8 must not
+# drop or rename there is already covered elsewhere: React Native's rules keep
+# every NativeModule, whose @ReactMethod methods JS calls by name, and AAPT
+# keeps the application, activity and services listed in AndroidManifest.xml
+# (FCM, UnifiedPush and the headless JS task). Keeping the package whole on
+# top of that costs a few KB and leaves its class and method names readable
+# in a crash report from an obfuscated release build.
+-keep class com.anonymous.bulwarkmobile.** { *; }

android/gradle.properties

@@ -25,6 +25,11 @@ android.useAndroidX=true
 # Enable AAPT2 PNG crunching
 android.enablePngCrunchInReleaseBuilds=true
 
+# Shrink release builds with R8 (code) and the resource shrinker. Keep rules
+# for classes reached by reflection go in app/proguard-rules.pro.
+android.enableMinifyInReleaseBuilds=true
+android.enableShrinkResourcesInReleaseBuilds=true
+
 # Use this property to specify which architecture you want to build.
 # You can also override it from the CLI using
 # ./gradlew <task> -PreactNativeArchitectures=x86_64