Run InfoMediaManager callbacks on a single thread.

- Creates an `Executor` and a `Handler` that run on the same thread.
- Terminates the handler thread when the last callback is unregistered.

Before this change `InfoMediaManager` used to have 2 threads for
 callbacks from 2 classes:
- For `MediaRouter2` it used a background thread executor.
- For `MediaController` it used a default handler which is a main thread
handler.

Fix: 425977951
Flag: EXEMPT bugfix
Test: on a physical device
Change-Id: I6bc89b54882b62fe876f055032392f8d72a9df67