Remove pending callback from AttentionManager

a) PendingCalback -> CurrentCallback + isDispatched + isFufilled
b) Simplify the cancellation logic. Only AttentionService should issue
specific failure codes, Manager should only issue "CANCELLED" if can't
reache the service

Bug: 111939367
Test: manually verified the demo worked, working on both CTS and unit
tests

Change-Id: Ief63b01a958bd67badd4d7ed8df9749baabb16f4