Fix possible NPE in SyncHandler.onBootCompleted()

If onBootCompleted() was called a second time, it would have throw a
NullPointerException since the queue object would have already been
set to null by the previous call. This commit just makes such an
object behave as an empty queue instead of throwing the exception.

Change-Id: I7ef82ba1e2504ceb0a2c11694d32f0588db7d0af
Merge "Fix possible NPE in SyncHandler.onBootCompleted()" into pac-4.4