SF: Tie main thread priority to active display

Otherwise, the powering off of the outer display when exiting concurrent
mode causes SF to drop real-time priority until the next powering on of
either display.

Fixes: 271431077
Test: Logs during boot, power on/off, fold/unfold, concurrent mode.
Change-Id: I4f1590d7abf82977c1acd162605f578624bc9faa