Ensure that BOARD_VNDK_VERSION is current

BOARD_VNDK_VERSION controls which version of the VNDK is used for the
build. We only support compiling against the current VNDK, and we don't
support installing old prebuilt VNDK libraries yet, so ensure this
variable is set to "current".

Once we support installing old prebuilt VNDK libraries, we'll also need
to disable building modules that use the VNDK.

Test: build-aosp_arm.ninja is the same before/after
Test: Ensure there are no boards that set BOARD_VNDK_VERSION
Test: Set BOARD_VNDK_VERSION := invalid
Change-Id: Ic26c9f44e356a0734f522b4538cb866d5b901d43