DO NOT MERGE: Don't trust callers to supply app info to bindBackupAgent()

Get the canonical identity and metadata about the package from the
Package Manager at time of usage rather than rely on the caller to
have gotten things right, even when the caller has the system uid.

Bug 28795098

Change-Id: I62710b15bb601fdfedd68e32349168c10725eb45
(cherry picked from commit d85a4ed2)