Combine AlertingManager and HeadsUpBindController

Previously, it was difficult to tell when a notification was inflated
before trying to show the heads up, and we got around it with a (now
flawed) heuristic based off if a notification was just inflated and
it has a heads up view.

This issue stems from having two control classes that are not really
communicating with each other. By combining these classes into one
heads up controller, the logic becomes more readable and we can directly
put a callback to show the HUN when the HUN content is inflated.

Bug: 149028421
Test: HUN smoke test
Change-Id: Ibb75597cfe26ccaf7e4d5247dc69d69bdf421ee6