CameraService: Watch for foreground changes from AppOps

AppOps now sends notifications when an app loses its camera capability.
We should watch for these notifications and block clients when they
happen, assuming they're untrusted. Otherwise, an app that should not
have camera access outside of the foreground may retain it.

Bug: 290086710
Test: Ran on physical device, tested Zoom + GCA
Change-Id: Ie69f96d91ec3898114a2c106afe3d42143cd8d33