Update vertical drag handle (all apps arrow)

- Change drawable to match specs, using ShadowDrawable to
  add shadow when necessary based on workspace theme.
- New drawable is 18dp by 6dp; add support for different
  width vs height, and decouple from workspace page
  indicator (which is still 24dp tall).

Bug: 151768994
Change-Id: Icfd0eac197ebc4d1f5bb799f8538c4bd99d800cd