Ensure runtime-visible annotations are kept

With R8 full mode, these annotations must be explicitly kept as
attributes. Also keep annotation defaults as they may be used at
runtime.

Test: m + presubmit
Bug: 215530220
Change-Id: Ibab630cd8498c5789ebcad6785fda24967df4d61

core/proguard_basic_keeps.flags

@@ -2,6 +2,11 @@
 # that isn't explicitly part of the API
 -dontskipnonpubliclibraryclasses -dontskipnonpubliclibraryclassmembers
 
+# Annotations are implemented as attributes, so we have to explicitly keep them.
+# Keep all runtime-visible annotations like RuntimeVisibleParameterAnnotations
+# and RuntimeVisibleTypeAnnotations, as well as associated defaults.
+-keepattributes RuntimeVisible*Annotation*,AnnotationDefault
+
 # For enumeration classes, see http://proguard.sourceforge.net/manual/examples.html#enumerations
 -keepclassmembers enum * {
     public static **[] values();