Fix mistake in disting of api txt

When disting into the apistubs/... dir (the "new" path),
the merged api txts are called android.txt rather than
current.txt, to align with all the other <module>.txt.

Also add a new dist goal to build just these files into a
convenient location.

Test: m api_txt dist
Change-Id: I762763cc42b497e00277a8871ef62b0a11423e35