Refine VisibleForTesting proguard rules

Both androidx (Jetpack) and com.google.common (Guava) define
@VisibleForTesting annotations. These are used within these libraries
for test purposes, but have also been adopted throughout the platform
for platform-specific test code. Consequently, the current keep
rules end up keeping not only platform test code, but also library test
code that isn't actually needed.

Refine these rules by restricting VisibleForTesting-annotated keep
rules to include code *outside* of Jetpack/Guava. A longer term
solution will involve migrating away from these annotations toward
the platform-defined @TestApi and internal @VisibleForTesting variants.
Note that platform code using the androidx* and c.g.common*-defined
@VisibleForTesting annotations will still be kept.

This saves about ~11MB of dex code from platform apps/services.

Bug: 239961360
Test: m + validate dex changes for shrunk packages (SystemUI/Launcher)
Change-Id: Icd70090e615b72a2cfb9ce8c43edb17cb46e05e2

core/proguard.flags

@@ -9,10 +9,15 @@
 # Add this flag in your package's own configuration if it's needed.
 #-flattenpackagehierarchy
 
-# Keep classes and methods that have the guava @VisibleForTesting annotation
--keep @**.VisibleForTesting class *
--keepclassmembers class * {
-@**.VisibleForTesting *;
+# 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.
+# 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.**,* {
+    @**.VisibleForTesting *;
 }
 
 # Keep rule for members that are needed solely to keep alive downstream weak