Make ClearKeyDrmUnitTest 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 ClearKeyDrmUnitTest with BOARD_VNDK_VERSION=current
Bug: 37342627
Change-Id: I5e95a19700522d3ec03e457d6779a34e9be99139