prebuilt_etc: Refactoring

* Replace `android.(Bool|String)` with `proptools.(Bool|String)`
* Remove `PrebuiltEtc.DepsMutator` and move the property check to
  `PrebuiltEtc.GenerateAndroidBuildActions`, so the property user is the
  checker.
* Add description to `PrebuiltEtcModule` interface.
* Wrap some very long comments.

Bug: 173379413
Test: TH presubmit
Change-Id: I3bb1375ff2b04909221a277ee0b384e2f5eeb42a