Fix crash in emulator unit tests

We strengthen the invariants in the async manager to make sure a task is
not running when it is being cancelled.

Without this it becomes incredibly difficult to guarantee that a task
is:

- Not actively running
- Scheduled to be called

This makes sure that we do not "delete" the TestModel while a task is
actively running.

Test: Additional unit test to validate expected behavior
Bug: 232262266
Change-Id: I6decf29193b0ee041b1d64a14de62b63528d223a