Rename le_audio namespace to bluetooth::le_audio

- both namespaces already exist concurrently and this
  is adding confusion to the code
- this will enable easier logging interface, removing
  the need for using namespace bluetooth in many places

The change was operated by regex application:

- namespace le_audio -> namespace bluetooth::le_audio
- (?<!bluetooth::|audio::|aidl::|hidl::)le_audio:: -> bluetooth::le_audio::

Manual corrections were required for the following cases:

- LeAudioClientInterface exists in the namespace bluetooth::le_audio and
  bluetooth::audio::le_audio
- The configuration provider relies on a generated flatbuffer backend
  in the namespace bluetooth::le_audio; it is renamed to
  bluetooth::fbs::le_audio to avoid name conflicts

Test: m com.android.btservices
Bug: 327453045
Flag: EXEMPT, mechanical refactor
Change-Id: I4eb51aca3a18ab6173aeebf82e44d5cd8b34a17e