cleanup: Fix permission protection of setObservedMotionEventSources

The previous permission protection was done inside a Binder clear
identity call, meaning that it used the permissions of system_server
instead of the permissions of the calling AccessibilityService.

Bug: 419110583
Test: atest AccessibilityServiceInfoTest
Flag: EXEMPT security bugfix
Change-Id: If64838388fa31bdc9abb0896d4011bfef8501a7c