Add TransitionHelper wrapper to handle the transitions API

Known issues:
- b/182705033 flashing screen when an activity finishes itself in
onCreate
- b/179340261 UI missing/non-interactable - had to implement
hack where we start activities inside a Handler#post.

Bug: 174335145
Test: manual
Change-Id: I2cca92c647595faffec06b01cb48db7509501bf4