Fix potential crash when TaskFragmentOrganizer died

Before, when TaskFragmentOrganizerController#onTransactionHandled()
is called, we will always call #validateAndGetState(), which call throw
an exception for racing condition like organizer process died.

Bug: 207070762
Test: atest WmTests:TaskFragmentOrganizerControllerTest
Change-Id: I04d37ecab4a37a616167ce4e6ca98c668f1254da