Check if rename of backed up file fails before persisting new changes.

If not these system services will end up with inconsistent settings files
when the device runs out of storage.
Delete mangled settings file in PackageManager if the current write fails
so that we don't end up overwriting the backed up version with the
mangled version
Include null check when retrieving fwd locked resource for an existing package

services/java/com/android/server/am/UsageStatsService.java

@@ -381,7 +381,10 @@ public final class UsageStatsService extends IUsageStats.Stub {
             mFileLeaf = getCurrentDateStr(FILE_PREFIX);
             // Copy current file to back up
             File backupFile =  new File(mFile.getPath() + ".bak");
-            mFile.renameTo(backupFile);
+            if (!mFile.renameTo(backupFile)) {
+                Log.w(TAG, "Failed to persist new stats");
+                return;
+            }
             try {
                 // Write mStats to file
                 writeStatsFLOCK();