Move the handling of carrier app changes in AppRestrictionController

...to non-bg handler.

As the bg thread is pretty slow and it may result in race conditions
if the carrier app requests to start FGS while the handling hasn't
be done yet.

Also fixed the test failures in BackgroundRestrictionTest.

Bug: 331282712
Test: atest BackgroundRestrictionTest
Change-Id: I9a3bcb576185b5859db58222906e86aacba6cb47
(cherry picked from commit 75a9c0f3)