Allow the down volume key to silence notifs

Additionally preserve existing behavior by passing
the keypress on.

Fixes: 130025623
Test: manual
Change-Id: I00f83275000369c1ab03bae63eb0a4b6170c4b53
(cherry picked from commit deb2d0b743e14871b964b30ad791ac5ff0cde016)
diff --git a/core/java/android/app/INotificationManager.aidl b/core/java/android/app/INotificationManager.aidl
index e57738f..9f51db8 100644
--- a/core/java/android/app/INotificationManager.aidl
+++ b/core/java/android/app/INotificationManager.aidl
@@ -108,6 +108,8 @@
     ParceledListSlice getNotificationChannelsBypassingDnd(String pkg, int userId);
     boolean isPackagePaused(String pkg);
 
+    void silenceNotificationSound();
+
     // TODO: Remove this when callers have been migrated to the equivalent
     // INotificationListener method.
     @UnsupportedAppUsage