Check cross user permissions for a given UID

Instead of relying on Context#checkCallingOrSelfPermission, explicitly
check permissions against a given UID. However, to maintain legacy
behavior, replace custom UIDs with Binder.getCallingUid when enforcing
permissions from a method.

Also update tests afftected by this change - by adding methods to mocked
objects

Additionally,
This reverts commit 7d0da763.

Change-Id: Ib31cabff5714500471bd397c743e127c85751a5c