FakePermissionEnforcer: provide a revokeAll method

This is useful in test suites that grant permissions on individual
tests, and then revoke all permissions in an @After method.

Bug: N/A
Test: TH
Change-Id: Ic3445f283702c7092c224bbe6f2340f5911b0541

tests/utils/testutils/java/android/os/test/FakePermissionEnforcer.java

@@ -47,6 +47,10 @@ public class FakePermissionEnforcer extends PermissionEnforcer {
         mGranted.remove(permission);
     }
 
+    public void revokeAll() {
+        mGranted.clear();
+    }
+
     private boolean granted(String permission) {
         return mGranted.contains(permission);
     }