...and now fail conservatively when two apps both attempt to define
the same permission. Apps signed with the same certificate are
permitted to redefine permissions.
We also finally have a (hidden) interface class for observing package
installation so that we can now rev the interface without breaking
existing callers.
Bug 13551375
Change-Id: Ifa4e59154dcccbb286ee46a35a6f25e4ad0f0f01