A2DP: Based on the HAL session type to control the encoder

Because we are now runtime switching the audio HAL between software and
hardare data path, it is not suitable any more to simply check whether
the offloading is enabled or not when handling the AV state change, and
here we check whether the current HAL is running under offloading.

TODO: BluetoothA2dp@1.0 is deprecated, and there would be UIPC and
BluetoothAudio@2.0 after related code removed.

Bug: 143922003
Test: Switching A2DP active devices and codec configurations
Change-Id: Iaa35013aa9dd4c928437b68bfa317696b8fad633