Retain all stubs versions in the snapshot.

This is a quick fix to avoid build failures due to prebuilts lacking
stubs versions that the sources have, which causes dependencies on
older version stubs fail on master-art-host where the sources of e.g.
Bionic are no longer available.

More work is required to make the version handling of stubs DTRT - a
TODO comment outlines it.

Test: m nothing
Test: `m runtime-module-sdk`, check that the generated Android.bp
  has the full stubs.versions list like the sources.
Bug: 169373910
Change-Id: I0ebfaf94f9d45a0e58d9785c40e7cea626f0ab83