Implement fake vendor snapshot

A fake vendor snapshot is a vendor snapshot whose prebuilt binaries and
captured headers are all empty. It's much faster to be built than the
real vendor snapshot, so users can exploit the fake vendor snapshot to
reduce the size of vendor snapshot they need, by installing the fake
snapshot and then inspecting the ninja dependencies.

Bug: 157967325
Test: m dist vendor-fake-snapshot
Change-Id: I7fa5d5033a296965b21e840765cc7fe02fd1f44e