Further refine VisibleForTesting rules

Additional analysis revealed more edge cases where external library
code was getting kept unnecessarily. Further refine the keep rules to
avoid these cases. Now, we only apply keep globally for the
`com.android.internal.annotations.VisibleForTesting` annotation.

VisibleForTesting annotations defined externally (e.g., by androidx or
guava) will *only* be respected in platform-defined packages. This
helps trim unused code from deps like gRPC and Dagger, saving up to
~hundreds of KB per package that uses these libraries.

Bug: 248580093
Test: m + verify exclusion of external test code (e.g., from dagger)
Change-Id: Iab7559c08d3ae1ac74f18e3cf3a1b4828a3736cf
diff --git a/core/proguard.flags b/core/proguard.flags
index 53f63d8..d790061 100644
--- a/core/proguard.flags
+++ b/core/proguard.flags
@@ -9,14 +9,19 @@
 # Add this flag in your package's own configuration if it's needed.
-# Keep classes and methods that have @VisibleForTesting annotations, except in
-# intermediate libraries that export those annotations (e.g., androidx, guava).
-# This avoids keeping library-specific test code that isn't actually needed
-# for platform testing.
+# Keep classes and members with the platform-defined @VisibleForTesting annotation.
+-keep @com.android.internal.annotations.VisibleForTesting class *
+-keepclassmembers class * {
+    @com.android.internal.annotations.VisibleForTesting *;
+# Keep classes and members with non-platform @VisibleForTesting annotations, but
+# only within platform-defined packages. This avoids keeping external, library-specific
+# test code that isn't actually needed for platform testing.
 # TODO(b/239961360): Migrate away from androidx.annotation.VisibleForTesting
 # and com.google.common.annotations.VisibleForTesting use in platform code.
--keep @**.VisibleForTesting class !androidx.**,!com.google.common.**,*
--keepclassmembers class !androidx.**,!com.google.common.**,* {
+-keep @**.VisibleForTesting class android.**,com.android.**,com.google.android.**
+-keepclassmembers class android.**,com.android.**,com.google.android.** {
     @**.VisibleForTesting *;