Cancels dual display state when all activities for process are invisible

Uses a ProcessObserver instead of a TaskStackListener to be notified
when a process has no foreground activities. This is a more stable
and correct signal than the onTaskMovedToFront notification.
cancel the dual display request. Fixes bugs previously seen with
tapping on navigation bars or multi-window scenarios.

Fixes: 295195310
Test: DeviceStateManagerServiceTest
Change-Id: I5aed2b204dcbdbf1e439811780a94b3f499b31a3