Prevent showing voice session when it is already queued to be hidden.

- It is possible for the session to be requested to be hidden before it
  gets the message to be shown and completes showing. This leads to an
  inconsistency where the voice interaction service implementation will be
  in a different state than the system for the session. Instead, we can
  cancel any pending show messages, and also clean up the pending show
  callback list immediately when the session is hidden.
- Also fixing up some error message codes when starting the assistant
  activity.

Bug: 38379130
Test: android.server.cts.ActivityManagerAssistantStackTests
Test: CtsVoiceInteractionTestCases
Test: CtsAlarmClockTestCases
Change-Id: I0d0e9c024367a47bda82d6a29ca89e18b7d69527