Various cleanup in soong_ui to aid new feature

- Rename the "BuildX" variables to "RunX"
- Remove redundant comments
- Inline all the "what to do" based on config in build.go
- Inline some constants only used in one place

Bug: 189187214
Test: m nothing
Test: build/soong/build_test.bash
Change-Id: I111a69e642212d7938d4971283545e0d9acbb01a