Detect (at runtime) kernel support for the "hdmi_audio" switch.

A new switch was introduced in AndroidAtHome to deal with a race
condition between the WiredAccessoryObserver and the HW composer HAL.
When the new switch ("hdmi_audio") is present, we want to pay
attention to it instead of paying attention to the old switch
("hdmi").  This change checks at startup for the presence or absence
of the new switch and uses it if available, otherwise it falls back on
classic behavior.

see change ID I960cfc2f3e8df5342e7248a26fd313fdad2ca322 for the kernel
side changes.

see bug 6023647 for a discussion of the issue.

Change-Id: I009e443f25662e7beb233e892ca71034b05ebfc2
Signed-off-by: John Grossman <johngro@google.com>