Autofill Fix race condition with onSessionDestroyed

Post onSessionDestroyed to the handler thread, to ensure that
FillEventHistory actions that are posted on the handler thread are run
before removing the session.

Fixes: 403642845
Test: atest android.autofillservice.cts.commontests.FillEventHistoryCommonTestCase
Flag: android.service.autofill.autofill_session_destroyed
Change-Id: I1cf0f2b8a08f585392beb15b13a29a3eb37d19a3