Taskbar motion polish to keep icons within taskbar bg bounds.

Instead of making the icons themselves move faster, we change
the startDelay/duration of the taskbar bg alpha animation
so that the taskbar is only visible when the icons are within
the taskbar bounds.

This is still an approximation. For U+, we should support taskbar
width change to fit the icons when they are in the 'hotseat'
alignment.

Bug: 273961611
Test: home <-> overview
      home <-> app
      stash <-> unstash
      foldable/tablet
      RTL
Change-Id: I39bedafac4afd641d250f7c97abf1e2de070d646