Rework activity launching and result handling

* Rename or eliminate methods that could be easily confused for AOSP
  methods due to the reuse of AOSP names.
* Separate out the handling of results for subactivites and for the
  next action (intent), each using its own activity launcher and
  callback.
* Add a StartDecoratedActivityForResult contract to ease the launching
  of activities without the caller needing to add SUW-related extras.

Change-Id: Iffaba4c51b2c90c42b8b243874a62cdea9e0c793