resolve possible Imf lock and a11y mLock deadlock

When IMF calls into A11yManagerService, A11yMannagerService
will post the call to main handler so the function call
returns without holding the a11y mLock.

Fix: 218182733

Test: atest AccessibilityImeTest

Change-Id: I3f7fcb1906b4084b2458ce2b65bb6fec8e9c6ae1