Animate depth from the right value on Taskbar All Apps launches.

`MyDepthController` in `QuickstepTransitionLauncher` assumes that we
want the background to always animate the same way, matching the rest
state of the workspace (depth == 0). However, in Taskbar All Apps the
background is visible, and depth != 0. We now initialize the one-off
`DepthController` for launches to take into account the latest depth set
by the top level `DepthController`, so there is no jumpcut at the
beginning of the animation.

Note that in my opinion we should use the same `DepthController` for all
cases, rather than having this one-off. I'm looking into the feasibility
of that change, but for now this fixes the issue at hand.

Fix: 292959100
Flag: N/A
Test: manual, see videos in the bug

Change-Id: Id90e8e728cc3e2ccf7d92148fbb0d6ff3e6fd6ca