fix($compile): ensure that `$onChanges` hook is called correctly

Due to the way that we instantiate controllers, the `$onChanges` hook
was not always available at the time we were trying to trigger the initial
call to this hook. For instance, if the hook was actually defined inside
the constructor function.

This commit fixes that but also fixes the fact that the initial call was
being made in the postDigest anyway, which was incorrect because the
it should have been made before the `$onInit` call.

Closes #14355
Closes #14359