Put back @interface in proguard flags

This was a workaround when @interface was not supported.
Bug: 27217036

Change-Id: Iaa96fd9ffc2a39994a3177caa8d9da5dcfc6c91e

proguard.flags

@@ -23,7 +23,7 @@
 }
 
 # Keep methods that have the @VisibleForAnimation annotation
--keep interface com.android.messaging.annotation.VisibleForAnimation
+-keep @interface com.android.messaging.annotation.VisibleForAnimation
 -keepclassmembers class * {
   @com.android.messaging.annotation.VisibleForAnimation *;
 }