Instead of having a module define `use_vndk: true`, assume that we're
building with the VNDK if we're installed on the vendor partition, and
BOARD_VNDK_VERSION==current. This now matches our behavior in Make.
Once BOARD_VNDK_VERSION!=current, we'll need to disable modules that
need to otherwise compile against the VNDK, since we can only compile
against the current VNDK.
Test: build.ninja for aosp_arm is the same before/after
Test: Ensure there are no boards that set BOARD_VNDK_VERSION
Change-Id: If937fa7bdb119648137af52daebadf486163484b
11b261472a