audio_hal_interface: Ensure that offloading_hal_interface is opened

..when get_a2dp_configuration is called.
To that end the reference to the a2dp_source message loop is
cached from the call to audio::a2dp::init
The offloading_hal_interface is re-opened if required in
the implemenetation of get_a2dp_configuration

Bug: 308686081
Bug: 305734815
Test: m com.android.btservices
Test: manual on cuttlefish: \
   disconnect and re-connect peer device from bluetooth settings
Change-Id: I92197f1472edea9c77042c6fad8dd8fbad5c23a4