Limit shared element transition to shared elements and children.

Bug 20257189

Shared element transitions should target the shared elements
or their children. When no target is given to a transition,
the transition should target the shared element. If a target
is given, it should also be able to target the children of
the shared element.

This is solved by wrapping the shared element transition
with a TransitionSet. The set is given the targets of all
shared elements and their children. Further, if no target
is given for a transition, the transition is given only
the shared elements as targets.

Change-Id: Ic015ca12e291ccbb47e20a49b11e3825da3e6e85