Fix RootScanner to execute scan at least once before cancelling it.

Previously RootScanner is implemented by FutureTask, and if it is
cancelled before it starts running in background thread, the scan will
be never executed.

The CL stops using FutureTask and introduces CountDownLatch to control
UpdateRootsRunnable.

BUG=27369585

Change-Id: Ica8799faba0a8e5ca91a6b8be36dc4f5118d6333