Rename ElementScenePicker to ElementContentPicker (1/2)

This CL is a pure refactoring to make ElementScenePicker also work with
contents, so that they can be used by overlays.

Bug: 353679003
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container
Change-Id: Iaf3a054170ec875c4c6a5f73b709aedab5c353e8