TouchExplorer: allow service to initiate touch exploration and there are no pointers on the screen

We need to be able to initiate touch exploration even if there are no pointers on the screen in order to reliably cover the case where a user just taps the screen.
The user expects to hear if they have tapped on anything.
Bug: 223673301
Test: manual. Tap on the screen and confirm that talkback reads the item in that position.

Change-Id: Id96fa8bd7f000ea153f45024e06915a43ca80826