Fix for a11yManagerService broadcastReceiver ANR

Offloads hard computations to background thread so broadcastReceiver can conclude before timeout

Bug: 333890389
Flag: aconfig android.view.accessibility.Flags.manager_avoid_receiver_timeout ENABLED
Test: add a long Thread.sleep() to the runnable and verify no ANR occurs
Change-Id: I64fe4e54c836a82ad028b908fae691b469a485d7
(cherry picked from commit b070e9ed)