prebuilt_etc: Improve comments and slight refactoring

Comment changes:
* Add top level package description.
* Clarify `OutputFileProducer` usage for `prebuilt_etc.src` field.
* Clarify output file basename selection rule.

Refactor changes:
* Don't emit superfluous `LOCAL_UNINSTALLABLE_MODULE := false` lines.
* `for _, i := range list { entry.AddStrings(prop, i) }` =>
  `entry.AddStrings(prop, list...)`

Bug: 173379413
Test: TH presubmit
Change-Id: Iab90aea5ff2497525ebe1600d3ce5a480923760a