AudioService test: spy communication with AudioSystem for A2DP

  Add an adapter class to spy on AudioSystem methods used for
device connection / disconnection.
  Modify A2DP connection tests to verify that after a device
is made available, it doesn't get disconnected later.

Bug: 146544156
Test: atest AudioDeviceBrokerTest

Change-Id: I897dac774d4de2cb40f29ccba9cdbc3493b3ce01