Implement vendor snapshot

Vendor snapshot can be captured with "m dist vendor-snapshot". With
vendor snapshot and vndk snapshot, older version of /vendor and newer
version of /system will be able to be built together by setting
BOARD_VNDK_VERSION to past vendor's version.

Only vendor modules under AOSP are to be captured. In detail, modules
under following directories are ignored:
- cts/
- device/
- external/clang/
- external/llvm/
- hardware/, except for:
    - interfaces/, libhardware/, libhardware_legacy/, ril/
- vendor/

Bug: 65377115
Test: m dist vendor-snapshot
Change-Id: Ib5859e409f8ab281ca137cae21e9ea08abd8a0b1