Hearing Aid Service: Set HA to active device if it's the first device

* On some conditions, the user switches the audio path to Headset after
connecting two sides Hearing Aids. After it, the one side Hearing Aid
suddenly disconnects with some reason (ex: connection timeout), then
reconnect. By currently design, Hearing Aid Service will set it to
active device again.
* The patch changes the behavior. When the Hearing Aid connected,
Hearing Aid Service will set it to active device if it is the first
device. It can avoid the reconnection event due to unexpected
disconnection interferes current active device.

Bug: 123633757
Test: make, Toggle disconnect/connect HA from Bluetooth UI,
Pair other side HA when music play and audio path is Headset,
Forget/Repair other side HA when music play and audio path is Headset,
Power off/on one side HA when music play and audio path is Headset,
Power off/on two sides HA when music play and audio path is Headset,

Change-Id: Ide101c838ad8d8d759c8b1d06cd6ab1d24c9af44