Reuse the main depth controller for launch animations.

The two controllers don't own any unique state, other than the current
depth state. These two _should_ never diverge anyway, because there is
only one valid depth at any given time. By using the same controller all
the time we can enforce this invariant.

Everything else is basically just registered and unregistered listeners,
which the main controller already does properly and always has the same
state in that regard as the ad-hoc one that we're removing.

Finally we don't need to take care of any cleanup explicitly, as we did
before.

Bug: 293427436
Flag: N/A
Test: manual
Change-Id: If6ea68847a60254df76e806eac2679ae0415bfe0