UBMS.mRunInitIntent: Mark as immutable.

No production code relies on the mutability. Code in fakeandroid's
AlarmManager added in http://ag/3826144 relies on the mutability,
and backup/internal/PerformInitializeTask sets a (now immutable)
PendingIntent, but fakeandroid uses a different AlarmManager instance
that the real system so these should be unrelated (I think - reviewer,
please double-check my thinking).

Fixes: 170163281
Test: Treehugger
Change-Id: I3dd45cc270c2fd8ab9d4b3402edc6cc5d765fd5a

services/backup/java/com/android/server/backup/UserBackupManagerService.java

@@ -650,7 +650,7 @@ public class UserBackupManagerService {
                         context,
                         /* requestCode */ 0,
                         initIntent,
-                        /* flags */ 0,
+                        /* flags */ PendingIntent.FLAG_IMMUTABLE,
                         UserHandle.of(userId));
 
         // Set up the backup-request journaling