STL: Add defaultOverscrollProgressConverter in SceneTransitions [1/2]

You can now define the ProgressConverter either in the
SceneTransitionsBuilder or in the OverscrollBuilder.
The default one will be used only if it's not defined in the
OverscrollBuilder.

Test: atest ElementTest
Bug: 336710600
Flag: com.android.systemui.scene_container
Change-Id: Ied52d2e8d93208b43850a217f7613b0c5ddb79cb
5 files changed