Fix backup agent un-proguard due to package name change

Change-Id: Id03ae4f25b7c3351b008d1d9a5daa93157a7a89d
diff --git a/proguard.flags b/proguard.flags
index 8a4a14f..9272fe9 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -1,2 +1,2 @@
 -keep class com.android.providers.userdictionary.UserDictionaryProvider
--keep class * extends android.app.BackupAgent
+-keep class * extends android.app.backup.BackupAgent