soong_ui ninja.go: comment and refactor.

On top of improving the comments, I also refactored the status checker
to simplify and clarify its use case to be a ninja stuckness checker,
since it doesn't appear to have other purposes.

Test: TH presubmit
Bug: b/173474588
Change-Id: I2cf51a1ebf16071a24a1c13c06c7b1adf60256de