PiP: Hide Menu when Keyguard is showing.

This used to be done relying on activity lifecycle, which is no longer
the case. We will instead use Keyguard listener callbacks to hide the
menu.

Bug: 169086834
Test: Open PiP, show menu, lock screen - don't see menu on AOD anymore
Change-Id: If98158ae69f0155c22e0a88d3e47f95d15ddfa0f