Crop taskbar icons during stash/unstash animation.

When we stash, we crop the taskbar icons to the height of the
stashed handle.

In a follow up CL we will translate the icons so that at the
end of the stash animation, the icons will overlap with the
stashed handle for a more seamless handoff.

This will be good to get in first so that we can see if there is
any performance regression by running multiple outline
animations at the same time.

Test: stash/unstash observe cropping
Bug: 267806083
Change-Id: I19b05647a669c11376ac1d3267fd600131246be8