Add mechanism for a task's windows to be trusted overlays

- Exposes a method to set that a certain part of the SF hierarchy is
  trusted, and sets this state for tasks in PIP.

Bug: 191529039
Bug: 196389741
Test: Manual, try using permission dialog while PIP is active

Change-Id: I170cb5a7d22ef569eb36de21cc0bcbef60dd385e
Merged-In: I170cb5a7d22ef569eb36de21cc0bcbef60dd385e