Directory structure of the audio HIDL related code.

Run `common/all-versions/copyHAL.sh` to create a new version of the audio HAL
based on an existing one.

audio
|-- 2.0              <== core 2.0 HIDL API. .hal can not be moved into the core directory
|                        because that would change its namespace and include path
|-- 4.0              <== Version 4.0 of the core API
|
|-- ...
|
|-- common           <== code common to audio core and effect API
|   |-- 2.0          <== HIDL API of V2
|   |-- 4.0
|   |-- ...
|   `-- all-versions <== code common to all version of both core and effect API
|       |-- default  <== implementation shared code between core and effect impl
|       |-- test     <== utilities used by tests
|       `-- util     <== utilities used by both implementation and tests
|
|-- core             <== VTS and default implementation of the core API (not HIDL, see /audio/2.0))
|   `-- all-versions <== Code is version independent through #if and separate files
|       |-- default  <== code that wraps the legacy API
|       `-- vts      <== vts of core API
|           |-- 2.0  <== 2.0 specific tests and helpers
|           |-- 4.0
|           |-- ...
|
`-- effect           <== idem for the effect API
    |-- 2.0
    |-- 4.0
    |-- ...
    `-- all-versions
        |-- default
        `-- vts