Add UID APIs to PlatformCompat.

Needed for scoped storage (to be used from MediaProvider).
IsChangeEnabled returns true if there are no installed packages for the
required UID, or if the change is enabled for ALL of the installed
packages associated with the provided UID.

Bug: 138275545
Test: Test app
Change-Id: I6785f57e4fcfb92ed7529f4da0a6db6ffe8b3ad4