Adjust policy for "stale" volumes outside module.

We're not ready to commit to VolumeRecord in an API surface, so the
next best choice we have is to place the logic in MediaStore, which
is outside the Mainline module boundary.

This also has the benefit of giving partners control over exactly
what policy they want to use for expiring stale volumes.

Bug: 137890034
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I677e4bfa6d08e32775e02c323debbaa90acf632b