Add `vendor_available: true` to libbuildverison

This allows using libbuildversion in Cuttlefish, which builds many
binaries for both vendor and host.

On vendor executables, this gets the build id from the
ro.vendor.build.version.incremental property.

Bug: 206843934
Test: m -j
Change-Id: I9ab11b42de3f1febf282f107c3e137592097d3c0