Register xml parsing receiver on background thread

One of a11yManagerService's receivers performs xml parsing.
Since this can take a while, the receiver should be registered on the background thread.

Test: atest AccessibilityManagerServiceTest (you'll need to verify via printing or breakpoints that the receiver is on the right thread)
Flag: aconfig manager_avoid_receiver_timeout
Bug: 333890389
Change-Id: I1ee6617fe1a4471d251c7a6d4c52b82a788776b1