Add a DefaultElementContentPicker factory for movable elements

This CL adds a DefaultElementContentPicker factory that can be used to
create a static picker for movable elements. I decided to use the same
PascalCase that needs to be used with the other pickers for consistency.

Bug: 353679003
Test: Trivial change
Flag: com.android.systemui.scene_container
Change-Id: I07ba56ce2ea17f13f79ec87af1948011289423aa