Streamline package-installed handling by the Backup Manager

In particular, don't do O(asec_apps * installed_apps) work during the
broadcast receiver's operation.  On devices with many installed apps
and a large number of them moved to ASECs, this was causing the system
process to become unresponsive and the watchdog to fire -- which in turn
would initiate a restart loop, as the same package-installed broadcast
would then be issued again once the package manager rescanned the ASEC
containers, ad infinitum.  With this change, the expensive call to the
package manager is only made once rather than asec_apps times.

Bug 5850283

Change-Id: I14e280ea1fa6af19cebc58869a20fbb599c92c8c