Divorce disting from androidmk

Change AndroidMkEntries.getDistContributions and
AndroidMkInfo.getDistContributions into a freestanding
getDistContributions.

This enables us to not call the AndroidMk related functions in
soong-only builds.

Bug: 398938465
Test: Diff'd the ninja files from "m nothing dist" before/after this cl, no changes
Change-Id: I2f9d537c0d66f0ae3715f083e2f55436e4c0a59f