Add DistPath to reference the dist folder

Instead of open-coding the logic of whether there is one, or where to
find it.

Bug: 76168832
Test: diff out/soong/build.ninja without dist
Test: diff out/soong/build.ninja with dist specified
Change-Id: Ia3f1ef335e2d6e2175343338d04867d778a50300
Merged-In: Ia3f1ef335e2d6e2175343338d04867d778a50300
(cherry picked from commit bc0c5092)