Extend SyncService with statFile public method.

1. Add public SyncService.statFile method.
2. Make doPushFile use source file lmt instead of current time.

Change-Id: Iaed45783dfd22dd095d2ee8ecb0c9f82100c21fa
(cherry picked from commit 9bd4c47f78635d3ac4686891ccf73f0a973b3dc4)
1 file changed