Use shell transitions to finalize cancel-to-desktop

When a to-desktop transition is cancelled, a WCT changing the task's
windowing mode to fullscreen was applied using #applyTransaction,
which meant that our transition observer doesn't get notified of the
change that allows it to swap the window decoration from the desktop
to the fullscreen version.
onTaskInfoChanged usually covers cases that transitions don't, but in
this case TaskListeners may just get a vanish callback for the freeform
trask and an appear callback for the fullscreen task. To avoid mixing
these two channels this CL changes the WCT to be applied using shell
transitions. It's also likely that it'll need to be animated in the
future to prevent a jump cut from one config to the next.

Bug: 290050157
Test: drag from fullscreen to desktop without releasing, drag back to
top to cancel the transition. Verify the caption version is correct.

Change-Id: I87ffaab5fc1b88a58b3d075a341a1e2a204a34cb