Make system/bt 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 system/bt with BOARD_VNDK_VERSION=current
Bug: 37342627
Change-Id: Ie1507e9b5bdb626fa0049faadd225c2c1f451661
Merged-In: Ie1507e9b5bdb626fa0049faadd225c2c1f451661
(cherry picked from commit 093123bf)