Add basic VNDK support in Soong

Export a list of libraries in the VNDK, which is currently empty.

Take in Make's global BOARD_VNDK_VERSION, and use that as the SDK
version for modules that specify use_vndk: true.

Modules that use the vndk have some configuration as if they were
building against the NDK (the absence of globally defined headers), but
in other cases look like platform modules (using the platform libc++,
for now).

This change does not attempt to enforce any linking constraints, that
will come in a later patch.

Test: out/soong/build.ninja doesn't change
Change-Id: I3be206b67015ac5177b7eef4a451c579e3dc903f