Allow granting permissions based on signature in <allow-permission/>

This patch allows us to either specify a sharedUserId or a package
signature to use when granting the specific permission.

Change-Id: I8aed78d40316e0e94ac1bfefc7c4a3016a2a9a6b

Conflicts:
	services/java/com/android/server/pm/PackageManagerService.java