Add support for cross-activity scenes and transitions

* Add theme attributes for specifying a top-level TransitionManager
  for an activity window.

* Add window feature for automatic content transitions. This
  automatically assigns/creates a Scene for setContentView calls.

* Add named transitions. This allows apps to define APIs for
  handshake-agreements about which exit/entrance transitions to play.

* Add new transition type for ActivityOptions. This lets the system
  use ActivityOptions to communicate transition specifics and
  arguments to the called activity.

* Have ActivityManager pass appropriate ActivityOptions through to the
  called Activity. Have the called activity call back into the caller
  to let it know which transition of a possible requested set was
  chosen.

Still to do:

* Define and pass arguments for transitions. This will require
  defining a Parcelable version of TransitionValues and deciding how
  much leeway apps should have for these things.

* Determine how to appropriately filter the ActivityOptions bundle so
  that only appropriate data reaches the target.

* Determine if generalizing the auto-Scenes functionality to
  ViewGroups is appropriate.

Change-Id: I10684b926129ab2fbc1adec9ef31767237acae79