Add conditional unit testing for codecs that might not be on the device

Codecs like aptX and aptX-HD are supported on the device if the
corresponding shared library is installed.
Fix the unit tests to conditionally test for aptX/aptX-HD.

Also, update function descriptions in files a2dp_sbc.h and
a2dp_vendor_ldac.h .

Test: Running unit tests on a device that doesn't have aptX/aptX-HD
Change-Id: I44dc275ca6e0cb5438ee6ed0b6213213d56592f4