Use getMaxConnectedAudioDevices when initializing the native stack

This change uses the system configured value for maximum connected audio
devices to inform the native stack how many A2DP Sink devices we're
allowed to have connected. The plumbing was added so the value is no
longer fixed to 1. This allows us to easily configure device count and
enable multiple connected audio devices.

The code to setActiveDevice has also been updated a bit to match what we
do in A2DP Source, allowing us to track and display the active device
for debug purposes.

Tag: #refactor
Bug: 180039146
Test: atest BluetoothInstrumentationTests
Change-Id: I890b542560907b9e1eb3a41a2b853240240b7bb5