Build libmockcasplugin 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: BOARD_VNDK_VERSION=current m -j k libmockcasplugin

Bug: 37342627

Change-Id: I6129ae91458216f537f6fb4065ecd9f26739851c