Audio V4: Fix documentation and useable/untestable bugs

Add missing documentation, especially for failure cases
and methods that are optional.
Add missing operation status for testability.
Add format for getSupported* methods as they could not
be implemented in 2.0 for multi-format stream (HDMI).

Test: compile
Bug: 38184704
Change-Id: I14ee5e02ef70a82df6de732f88b065810e9b032f
Signed-off-by: Kevin Rocard <krocard@google.com>