Merge "Add a callback to listen to uid foreground-ness change." into udc-dev am: b0302b10

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/22499498



Change-Id: I4111b424891a2d1f22b39b44954d19b6c587831d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>