Keep methods annotated with @JavascriptInterface

This rule is a common default rule elsewhere, and should be applied
globally for platform targets rather than relying on per-target rules.

Follow-up work will remove duplicated rules from individual targets.

Bug: 248580093
Test: m
Change-Id: If3cb9fb22d2acf90d87f150f93ad491549bf9090

core/proguard_basic_keeps.flags

@@ -41,6 +41,11 @@
     java.lang.Object readResolve();
 }
 
+# Keep all Javascript API methods
+-keepclassmembers class * {
+    @android.webkit.JavascriptInterface <methods>;
+}
+
 # Keep Throwable's constructor that takes a String argument.
 -keepclassmembers class * extends java.lang.Throwable {
   <init>(java.lang.String);