Don't collect redundant activities in transient launch

Transitions shouldn't collect any changes they don't actually
modify. This isn't a big problem now, however, once we have
simultaneous transitions, this will force transitions to
serialize when they aren't actually interdependent.

This was collecting the activities only for the purposes of
tracking what to commit in finish. So instead, just calculate
the commit targets in finish.

Bug: 264536014
Test: atest TransitionTests#testTransientLaunch
Change-Id: I4eb34adefb35ad001e6ef2734b10e9d5aacac611