Allow to specify transitions between contents

This CL is a small refactor of the transition DSL API so that
transitions can now be defined between two contents rather than two
scenes.

Bug: 353679003
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: com.android.systemui.scene_container
Change-Id: Iab1d57563010c2a66b6ed759f2d9ac9cd18300e5