Run pending-intent-sent callback on UI thread.

Before this CL the callback passed to
StatusBar.startPendingIntentDismissingKeyguard() was being run on a
background thread otherwise used to run the pending intent. This is not
obvious to the caller of startPendingIntentDismissingKeyguard(), so with
this CL we call the callback to the UI thread and make it clear that's
the case.

Bug: 120894295
Test: atest SystemUITests
Test: manual - ensure HUN is removed when pressing HUN smart action.
Change-Id: If106b697527843dd74e37d14de882d5f08187285