Remove lifecycle code and warnings from constructor in ShadowActivity

We removed the code in the `ShadowActivity` constructor that faked out part of the `Activity` creation lifecycle and issued a warning to the user to use `Robolectric.buildActivity`. This change prompted many fixes to Robolectric test code that was relying on this behavior. Instances were replaced with `Robolectric.application` where appropriate. When an actual `Activity` is needed, `buildActivity()` is used with a `create()`.

This change has the potential to break folks that are not using `buildActivity`. We hope that they have had enough time (since late May or 2.1) to migrate to the `ActivityController` API. Other changes we are working on (#659 in particular) will also render the code in the `ShadowActivity` constructor problematic, so we think it's time to kill this code.