Directory structure of the audio HIDL related code.

audio
|-- 2.0              <== HIDL (.hal) can not be moved to fit the directory structure
|                        because that would create a separate HAL
|-- 4.0              <== Version 4.0 of the core API
|
|-- common           <== code common to audio core and effect API
|   |-- 2.0
|   |   |-- default  <== code that wraps the legacy API
|   |   `-- vts      <== vts of 2.0 core and effect API common code
|   |-- 4.0
|   |   |-- default
|   |   `-- vts
|   |-- ...          <== The future versions should continue this structure
|   |   |-- default
|   |   `-- vts
|   `-- all_versions <== code common to all version of both core and effect API
|       |-- default
|       `-- vts      <== vts of core and effect API common version independent code
|
|-- core           <== code relative to the core API
|   |-- 2.0          <== 2.0 core API code (except .hal, see audio/2.0)
|   |   |-- default
|   |   `-- vts
|   |-- 4.0
|   |   |-- default  <== default implementation of the core 4.0 api
|   |   `-- vts      <== vts code of the 4.0 API
|   |-- ...
|   |   |-- default
|   |   `-- vts
|   `-- all_versions
|       |-- default
|       `-- vts      <== vts of core API common version independent code
|
`-- effect           <== idem for the effect API
    |-- 2.0
    |   |-- default
    |   `-- vts
    |-- 4.0
    |   |-- default
    |   `-- vts
    |-- ...
    |   |-- default
    |   `-- vts
    `-- all_versions
        |-- default
        `-- vts