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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:5673af6bd01f27f50768e8e2d5b7d31cea461f1e)
Merged-In: Iaa21d74873da1dcc2453ad98abb284bad3a211f5
Change-Id: Iaa21d74873da1dcc2453ad98abb284bad3a211f5