Generate VNDK snapshot zip as a build artifact

Define a new phony target 'vndk' such that 'make vndk dist'
outputs VNDK snapshot zip file to DIST_DIR.

Bug: 66059995
Test: m -j vndk dist TARGET_PRODUCT=aosp_{arch}_{a, ab}
BOARD_VNDK_VERSION=current
Change-Id: I6fd763a91205b21fba414d1f3068408a6826fad8