Parse preferred-apps for all system partitions

Previously only /system/etc/preferred-apps was parsed. This reads
PackageManagerService.SYSTEM_PARTITIONS instead so that these can
be configured in any partition.

Bug: 149779143

Test: manual device boots, works as expected

Change-Id: I7e63aaaa3d050ee8f7be85e2e557bd1a2b88fd1b