Relocate transition collection around lifecycle events (1/N)

Previously was just using whatever the legacy transition system
used. However, that system would wait too late (until after
round-trips to the client) in order to prepare some transitions.
Since the new transitions utilize "snapshots", they need to
collect earlier in the lifecycle to work properly.

This first CL deals mostly with activity start/finish and
task to-front/to-back. It also adds some utilities like the
ability to abort a transition (needed because activitystarter
works this way) and the ability to un-ready a transition/
sync transaction to enable trampolines.

This also switches the transit types to the new constants.

Bug: 169035022
Test: Existing tests (TransitionTests) pass
Change-Id: I05f3a2e8e27ccaed7077d648daf033aeabc333f9