Simplify soong_build top-level code

* `android.Context` holds `android.Config`, so provide an accessor to it
  and avoid passing them separately to a lot of functions.
* factor common code in `readBazelPaths`/`getExistingBazelRelatedFiles`
  into `readFileLines`
* refactor check-error-and-quit into `maybeQuit`
* use switch instead of if-elseif-... where appropriate
* rearrange the code in `runApiBp2build`, `runSymlinkForestCreation`

Test: treehugger
Change-Id: I639929c21ec4999cdfd737c07136f32df2d488bc