hal: restrict 24 bit capture for unprocessed audio

- configure backend to 24 bit only when source is unprocessed
- for all other source force 16 bit only

- final result

  -----------------------------------------------------------------
  source        app request    audio policy requests    hal returns
  -----------------------------------------------------------------
  Unprocessed   16             8_24                     8_24
  Unprocessed   24/float/8_24  24/float/8_24            8_24

  any other     16             8_24                     -EINVAL
  (retry)                      16                       16

  any other     24/float/8_24  24/float/8_24            -EINVAL
  (retry)                      16                       16
  -----------------------------------------------------------------

Bug: 27348418.

Change-Id: I01139e741a414db1b9ff0079984cb5da007f9a0c