Add self-restore capability to Backup/Restore sample app

The Backup/Restore sample application now provides a "restore my own
last-known-good data" button.  When it is clicked the app will run the
standard public BackupManager.requestRestore() operation.

(This was the one aspect of the public API that was not previously
exercised by the sample app.)

Change-Id: I8abcfbad4b27b35fe9fafbbb97f89bac1d7a668c
diff --git a/samples/BackupRestore/Android.mk b/samples/BackupRestore/Android.mk
index c164a6c..abe2526 100644
--- a/samples/BackupRestore/Android.mk
+++ b/samples/BackupRestore/Android.mk
@@ -10,4 +10,6 @@
 
 LOCAL_SDK_VERSION := current
 
+LOCAL_PROGUARD_FLAG_FILES := proguard.flags
+
 include $(BUILD_PACKAGE)