Expose the HomeVisibilityListener API

Rename HomeVisibilityObserver to HomeVisibilityListener and expose it as
module-only API. This listener is used by the SIM toolkit to detect when
the home screen is visible.

Also fix some bugs in the original implementation that were uncovered
during CTS writing.

Fixes: 170226810
Test: atest ActivityManagerTest#testHomeVisibilityListener
Exempt-From-Owner-Approval: Previously approved in ag/12775889
Change-Id: Ia44462e55ad71119cf356485c60c121d331c50d4