ci(android): gate name-based reachability on an R8-minified variant

R8 only ever ran on `release`, so every automated gate in this repository
exercised code the shipped APK does not contain. Reflective lookups, JNI
callbacks and native library loading can all break under shrinking while
`flutter test`, the Robolectric suites and `connectedDebugAndroidTest`
stay green — which is exactly how #1703 shipped, with the bundled FFmpeg
audio renderer shrunk out of release builds for TrueHD and DTS-HD.

Add a `minified` build type that inherits release's shrinker
configuration but stays debuggable and debug-signed, so it is an ordinary
test artifact and never a publishable one. Three integration details
took a run each to find: the Flutter plugin copies app build types into
every plugin module, so library-level shrinking deleted the plugin entry
points that only GeneratedPluginRegistrant references; the harness must
not be shrunk or the runner disappears; and androidx.test has to survive
in the app under test, or the runner cannot link its own supertype and
the run reports zero tests instead of failing.

Instrumentation still defaults to `debug`, because only one build type
can host androidTest and the existing playback suites drive media3
builder APIs the app never calls, which R8 shrinks legitimately. The new
reachability test opts into the minified variant instead and touches no
builder API, so the only keeps it depends on are the ones under test.
Emptying proguard-rules.pro was verified to fail it.

.github/workflows/e2e.yml

@@ -197,6 +197,22 @@ jobs:
             -no-boot-anim -camera-back none
           script: python3 scripts/run_maestro_ci.py android-15-instrumentation
 
+      - name: Run Android R8 reachability
+        id: api35-r8
+        continue-on-error: true
+        uses: reactivecircus/android-emulator-runner@a421e43855164a8197daf9d8d40fe71c6996bb0d # v2
+        with:
+          api-level: 35
+          arch: x86_64
+          profile: pixel_6
+          avd-name: maestro-api35-instrumentation
+          force-avd-creation: false
+          disable-animations: true
+          emulator-options: >-
+            -no-snapshot-save -no-window -gpu swiftshader_indirect -noaudio
+            -no-boot-anim -camera-back none
+          script: python3 scripts/run_maestro_ci.py android-r8-reachability
+
       - name: Run Android 15 suites
         id: api35
         continue-on-error: true
@@ -299,9 +315,10 @@ jobs:
           retention-days: 7
 
       - name: Report suite failures
-        if: ${{ always() && (steps.api35-instrumentation.outcome != 'success' || steps.api35.outcome != 'success' || steps.api28.outcome != 'success') }}
+        if: ${{ always() && (steps.api35-instrumentation.outcome != 'success' || steps.api35-r8.outcome != 'success' || steps.api35.outcome != 'success' || steps.api28.outcome != 'success') }}
         run: |
           echo "Android 15 instrumentation outcome: ${{ steps.api35-instrumentation.outcome }}"
+          echo "Android R8 reachability outcome: ${{ steps.api35-r8.outcome }}"
           echo "Android 15 outcome: ${{ steps.api35.outcome }}"
           echo "Android 9 outcome: ${{ steps.api28.outcome }}"
           exit 1