Moved system user apps whitelisting to PM

Previously applications where uninstalled for the system user at a later stage
in a boot process, when PM has already been initialized and registered as
a service. This was causing a flood of PACKAGE_REMOVED broadcasts.

Applications are now uninstalled at an earlier stage, when PM hasn't been
registered. Packages can simply be marked as not installed and broadcast
can be skipped.

Bug: 25626819
Change-Id: Ib64e7c45619809b0efb99cc566a9a2120ed48cd3