Handle onUnlockUser on fg thread

AppWidgetService.onUserUnlocked can block ActivityManager thread
for 500+ ms. The code is guarded by mLock, so it's safe to simply
offload it to the worker thread without additional synchronization.

Test: Setup widgets/rebooted the device and unlocked
Test: Create/switch to guest user + setup widgets
Bug: 63389529
Change-Id: Ib3bc76f79bdbc707c0ac270d7fc35c8b27af9af0