Allow app property to pass through addon proxy to addon instance

[raycohen]

When the EmberApp constructor runs, the presence of nested addon dependencies can result in the project.addons list containing a mixture of addon instances and addon proxy objects. For the addon proxy objects, the EmberApp setting their app property would prevent the addon instance from having it set, which goes against the expectation described in the models/addon.js app property comment:

ember-cli/lib/broccoli/ember-app.js

Line 132 in ee6d372

this.project.addons.forEach((addon) => (addon.app = this));

ember-cli/lib/models/addon.js

Lines 188 to 193 in ee6d372

	/**
	The host app instance.
	**Note**: this property will only be present on addons that are a direct dependency
	of the application itself, not of other addons. It is also not available in `init()`,
	but will be set before `setupPreprocessorRegistry()` and `included()` are invoked.

Later on, when an addon attempts to read this.app it will be undefined if the addon was a Proxy by the time the EmberApp constructor method attempted to set it. This seems like a bug.

The comment explaining the proxy holding the app claims that ember-cli might set that app property, but I'm not sure what situation it is referring to. I didn't see any test coverage of this behavior. Simply removing this fixes the issue where addons in my project that expect to be able to read this.app in their index.js functions were erroring.

[rwjblue]

@brendenpalmer - Would you mind reviewing?

[bertdeblock]

Anyone who can verify this change? Would be nice to get this in if this actually fixes an issue.