Restore profile saver priority in GC critical section

Restore profile saver thread priority during the GC critical section.
This helps prevent priority inversions blocking the GC for long
periods of time.

Average time spent in GC critical section during phone booting
(20 samples): 137.4963182ms -> 62.22603846ms

Bug: 62941975
Test: phone boot and make

Change-Id: Ie8180974e92a7bf5ebb847eeee741c7fd5bdf59f