libaaudio: remove LEGACY sharing mode

It was a poor API. Choosing the AudioTrack path is not a sharing mode.
Instead we decide internally, which path to choose based on sharing
mode and availability.

Test: CTS nativemedia/aaudio/test_aaudio.cpp
Change-Id: I8b6246b95cea807dcd0fc82e08a16f68f17b47f3
Signed-off-by: Phil Burk <philburk@google.com>