Add clearAll method to RegistrantList for easier cleanup

Bug: 155320231
Test: atest FrameworksTelephonyTests
Change-Id: I6f1896ae163e77cff30e89a5b01b41162fc58200

core/java/android/os/RegistrantList.java

@@ -66,6 +66,10 @@ public class RegistrantList
         }
     }
 
+    public synchronized void removeAll() {
+        registrants.clear();
+    }
+
     @UnsupportedAppUsage
     public synchronized int
     size()