PackageManager Handler Thread Priorities adjustment.

Primary handler thread priority changed to DEFAULT from BACKGROUND.
Secondary handler thread priority changed to BACKGROUND from DEFAULT.

There are a few write* operations which include IO, but they are
executed under mLock and we want them to complete ASAP.
If at some point they are made lockless then we will move them to the
secondary thread.

Bug: 230521658
Fixes: 230521658
Test: presubmit
Change-Id: I4d0c6ea696e31976216c69b9be10a8f3f5efd833