Add DistForGoal to MakeVarsContext

Add methods to MakeVarsContext to allow Singletons to dist
artifacts without manually adding $(dist-for-goals) in Make.

Bug: 153485543
Test: m checkbuild
Change-Id: Ia5ddb31afe29329f2df0ae1297ed963c8c28e590
Merged-In: Ia5ddb31afe29329f2df0ae1297ed963c8c28e590
(cherry picked from commit 3cda0d8d)