Make it possible to disable shared animations (1/2)

This CL makes it possible to disable shared element animations, that are
enabled by default when an element is shared between two scenes. This
can for instance be used to disable the sharing of notifications when
going from the Shade to Lockscreen, or disabling the sharing of media
player when going from lockscreen to shade on the phone while keeping it
on tablets without having to use different keys.

See b/300867076#comment3 for videos.

Bug: 300867076
Test: atest SharedElementTest
Change-Id: I30384e064eb66f81f39ba0185f35c375f2741590