Space out empty taskbar hotseat views when on home screen

This ensures the taskbar hotseat aligns with the home screen
hotseat, as the latter supports empty cells. When in apps,
the taskbar still collapses the empty cells.

Test: Turn off hotseat predictions, remove some hotseat items,
and ensure the taskbar hotseat spreads out when on home.

Bug: 179886115
Bug: 171917176
Change-Id: I6047c3c5691685edcd8b3519e0305812b1295550