Unify ember_debug's usage of `owner` vs `_application`

Currently, `EmberDebug` uses `owner` in our own tests, but in production we _always_ use `_application.__deprecatedInstance__`. This split is kinda bad to begin with but it also prevents issues with using the ember-inspector while running an Apps own tests.

Start by locating EmberDebug and its test and production references to owner and _application.__deprecatedInstance__; compare how each path obtains the application. Confirm that the chosen access pattern works both during an app's own tests and in normal production use, then run the relevant EmberDebug tests.

Written by the indexing model from the issue text.