Signal "Connecting" state when receiving audio focus

Problem: The main AAOS Media Center app relies on CarMediaService, who
does not listen to the media framework media key event session changed
callback to understand which application on the system the media
framework would forward key events to. Instead, they have their own set
of criteria for determining which application is the currently playing
application that should be shown in their UI and have key events routed
to it. Key to their method is the presence of an "active" playback
state, which can sometimes take a long time for our service to reflect
due to the remote device needing to establish playback first. As well,
sometimes devices don't establish playback at all due to bugs on their
side! This causes Bluetooth not to show up at the playing app in Media
Center, which also keeps the user from being able to use controller
side media controls to fix the error state.

Solution: Briefly indicate we're in the "connecting" state when we gain
audio focus. This state is considered "active" by the Media Center's set
of conditions and causes us to be shown immediately, placing Media
Center's UI in sync with the media framework's view of the world.

Flag: 350510879
Bug: 346831665
Bug: 310001181
Test: atest BluetoothMediaBrowserServiceTest
Change-Id: If113db4bb3fa780d3c35c9fbe8499d0076d0a143