Introduce soong only androidmk singleton

The newly introduced singleton runs soong only androidmk build actions
that were run inside the androidmk singleton. The singleton is
registered later than other singletons so that the dist contributions
from other singletons are collected.

Test: m droid dist --soong-only && ls out/dist
Bug: 394365683
Change-Id: Iba4592864ce46a7ed4007776fa1de5bc6ba54dc1