Make audio_extn build with the VNDK.

When the vndk is enabled (BOARD_VNDK_VERSION=current),
vendor libraries must only link against vendor variants
in the build system. This also means that they receive
restricted sets of headers and that automatic global
headers are restricted.

Test: build audio_extn with BOARD_VNDK_VERSION=current
Bug: 69574580
Change-Id: Iaae465a68d04bb991a95442a0114591849d282a3