Add keep rules for @KeepForWeakReference annotation

Ensure annotated fields are kept when treeshaking-related optimizations
are enabled. Note that this still allows field removal if tree shaking
reveals no references, but will avoid removal if there are references
that might impact downstream usage, e.g., weak references.

Bug: 227455445
Test: atest InternalAnnotationsTest
Change-Id: I59c6277c43b14c5de8154fa1eefad01f0702e413

core/proguard.flags

@@ -15,6 +15,12 @@
 @**.VisibleForTesting *;
 }
 
+# Keep rule for members that are needed solely to keep alive downstream weak
+# references, and could otherwise be removed after tree shaking optimizations.
+-keepclassmembers,allowaccessmodification,allowobfuscation,allowshrinking class * {
+  @com.android.internal.annotations.KeepForWeakReference <fields>;
+}
+
 # Understand the common @Keep annotation from various Android packages:
 #  * android.support.annotation
 #  * androidx.annotation