Introduce SharedElementScenePicker

This CL introduces the SharedElementScenePicker to allow transitions to
define in which scene a shared element should be drawn or composed
during the transition. This is for instance necessary when a shared
element is disappearing from one scene then reappearing in another, like
the media player when going from the Lockscreen to the Shade on phones.

An example of this API will be added in ag/25061287.

Bug: 291053742
Test: atest MovableElementTest
Change-Id: I44e01aa44c614ad5f3f1bb884b7d73ab34ab2774