Use grid border spacing as widget padding if possible.

Widget padding is added by the system to create reliable consistency
between widgets.

Now that we have added border spacing between cells on the launcher
grid, we have significantly decreased the available space for widgets
on home screens: widget padding is 8dp on all sides, border spacing is
currently spec'd to be 16dp.

We can inset the widget padding added by the system, and instead rely
on the border spacing between cells to create reliable consistency
between widgets.

Bug: 175329686
Test: manual
      turn CellLayout#VISUALIZE_GRID debug flag on
      test drag and drop, resizing using drag handles
      view measurements in go/web-hv
Change-Id: Iabb9f45e82e4ae039f55d7fd27fbd0d23be60191