Update taskbar to spec

- Space between icons is 24dp in all cases (transient, persistent,
  small tablet, large tablet)
- Persistent taskbar icon size remains 44dp, but transient taskbar icon
  sizes have changed slightly: 57dp -> 52dp for large tablets and
  50dp -> 48dp for small tablets
- Also moved all values to quickstep package to avoid confusion

Test: manual, visual
Fixes: 263465844
Change-Id: I563d64dca2d920762233dd6147395c5bba3ffcf3