STL: Default overscroll ProgressConverter slowly approaches 0.2f [1/2]

In some previous CL we introduced a new default ProgressConverter for
Flexiglass ag/28922176 and the Demo app ag/28945142.
This CL introduces the default for STL in general as well.

Before and after attached to b/336710600#comment20

Test: No test for the new default behavior
Bug: 336710600
Flag: com.android.systemui.scene_container
Change-Id: I43f014e7d6c659911a864bba861ba8da0d1dfd13