Implement missing getAssignedPackageIdentifiers delegate

Just leaving the implementation empty as that should avoid the crash
when calling it. The default stub was returning null and all the uses
were expecting an instance of SpareArray

Bug: http://b.android.com/211529
Change-Id: I497f823a6bfb7a6a946ba20c4f31b1020d2a0cef
(cherry picked from commit 98b704a284870b52cec37bf19370432c194e0608)