Expand install observer semantics

...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