Some clean up of app install and user management.

UserManagerService is now closely tied to PackageManagerService,
sharing the same locks.  There is no longer direct access of
Installer by UserManagerService, instead the package manager is
back to solely owning it.

Creating a new user now correctly only installs system apps for
that user.

Fixed some misc bugs, where we were getting nulls when
querying content providers and instrumentation in uninstalled
users, incorrect locking, etc.

Change-Id: Ife69b6e373d0cf7c5cfc03fc588e36b43ad5d8b0