Make ClearKeyFetcherTest 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 ClearKeyFetcherTest with BOARD_VNDK_VERSION=current
Bug: 37342627
Change-Id: If4a6b78f515a29c0af13ca1db8c75a06d10369eb