Reformats a SafeArea introduced in #13853.

Applies horizontal safe area insets to the animation demo in the
Gallery. Specifically, this ensures the back button is positioned
consistently with iOS expectations and that that main image card in the
detail view respects safe area insets.

This is to support the iPhone X sensor housing notch and other similarly
creative display features when in landscape orientation.

Alignment will eventually replace FractionalOffset.

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

* make @immutable const

* fix build

* add @required when there's an assert not null

* address review comments

* use color.shadeXxx instead of color[Xxx]

* remove calls to .shade500 on MaterialColor

* remove calls to .shade200 on MaterialAccentColor

* fix test

This machinery is useful for arbitrary boxes (e.g., with FittedBox).

Fixes #6463