fix($compile): Cleanup attribute-binding observers

[jonathantyates]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Bug fix

What is the current behavior? (You can also link to an open issue here)
#15268

What is the new behavior (if this is a feature change)?
Cleans up attributes observer on change.

Does this PR introduce a breaking change?
No.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[gkalpak]

The implementation looks good. The testcase doesn't seem to test the new change, though.

Off the top of my head, you can exercise the appropriate path by having a controller that returns an object from its constructor. E.g. something like:

function TestController() {
  return {
    $onChanges: ...
  };
}

TestController.prototype.$onChanges = ...

In thie above case, both $onChanges functions should continue to get called without the fix. With your fix, only the returned one should continue to get called onve the controller is initialized.

[gkalpak]

Nit: Rename to TestController.

[gkalpak]

LGTM 👍 _{(I will rename the controller while merging.)}

✨ Thx for finding the bug and fixing it too! ✨