proguard: add keep rules for @TestApi

Test: build
Bug: 319319578
Change-Id: I739a80c66de57d0ce68624246613722edf2d106f
diff --git a/core/proguard.flags b/core/proguard.flags
index 9cbba0f..aa406b9 100644
--- a/core/proguard.flags
+++ b/core/proguard.flags
@@ -15,6 +15,13 @@
     @com.android.internal.annotations.VisibleForTesting *;
 }
 
+# Keep classes and members with platform @TestApi annotations, similar to
+# @VisibleForTesting.
+-keep @android.annotation.TestApi class *
+-keepclassmembers class * {
+    @android.annotation.TestApi *;
+}
+
 # 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.