Refactor OWNERS files in platform/build/soong.

Pulls out all of the per-file rules into their relevant directories
so platform/build/soong:/OWNERS can be included everywhere we need
the list of build system owners.

Test: treehugger
Bug: 170407947
Change-Id: I5a4b2d252bd9b3c280cd9954cf0e65ac21153ede