Don't set mDecor on Activity until after create

This value needs to be set to the Window's decor view and this can not be accessed until
after onCreate is called or feature requests will fail within onCreate.
Update View attaching tests to be more realistic

This just switches the tests to use a View that follows a standard lifecycle
rather than setting the contentView after resuming.
Add 'visible' method on `ActivityController` for making the Activity visible

This method adds the Activity's window decor view to the window manager. This is required
for `View.isShown` to return true as otherwise the Activity's view hierarchy won't have a parent.

In real life this happens some time after the Activity resumes (in `ActivityThread`) so it really needs
to be its own lifecycle operation (even though its not part of the 'official' lifecycle).