Track changes during transitions and report to player

Current tracked changes include bounds, visibility, and
"existence".

bounds and visibility are fairly straightforward, call
collect() before doing operations and it will record the
state. Once the transition is finished, it will compare
that recorded state with the finished state.

"existence" is more complicated. It's the difference
between show/hide vs open/close. Because Task instances
are basically always around, their existence depends on
the circumstances around when/how activities are started.
For that we have to explicitly indicate when a container
is going to change its existence via
collectExistenceChange().

Bug: 169035306
Test: atest TransitionTests
Change-Id: I78d4cda6b726202507b757f93c742d49e51e2d7b