Add API to show background behind animating windows

For the new style of Material 3 activity transitions (go/t-activity-transitions) we fully fade in and out activity windows to a solid background color (that of the activity's theme). This means we need to provide a way for activity transitions to show a solid background color behind the animating windows to fill any space that would otherwise be black. This ability should be available both to system transitions but also to apps that want to create transitions in a similar style without necessarily using exactly the system one.

The API provides a way to specify if a background should be shown behind
the animation or not. If a background is requested it is up to the
transition handler to drawn this background appropriately based on the
type of transition and task/activity info.

Test: atest CtsWindowManagerDeviceTestCases:AnimationBackgroundTests
Bug: 202844659
Change-Id: Iddee6583fb1ad83e3ec238a89030b8c284e04d03