Make libstagefrighthw 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 libstagefrighthw with BOARD_VNDK_VERSION=current
Bug: 69526027
Change-Id: Icbf9dd22646a1173d1f97e7646a2ee2f4d143074