audio_hal_interface: Split off A2DP AIDL bindings

Move everything A2DP specific to system/audio_hal_interface/aidl/a2dp
Common definitions for the client interface and transport are duplicated
under the namespace bluetooth::audio::aidl::a2dp.

The common definitions will be tailored down to A2DP's usecase in a
follow up change.

Bug: 365022887
Test: m com.android.btservices
Test: Manual streaming test
Flag: EXEMPT, mechanical refactor
Change-Id: I32e5b447487fdde25845a62349db843262fa54d4