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

[rwjblue]

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.

[RobbieTheWagner]

@patricklx I'm unsure if this issue still needs to be addressed? I know you have heavily refactored a lot of this, so perhaps we can close this?

[patricklx]

it's still testing with owner in our own tests