Add special column for All Apps button in FocusLogic's sparse matrix.

The All Apps button creates a number of edge cases, mainly because it
causes the hotseat to sometimes have an extra column than the workspace.
Previously, we sort of swept these problems under the rug by simply
ignoring the All Apps button if other icons were present in the hotseat,
with the assumption that those other icons should get focus instead of
the All Apps button. (If possible, we want to stay in the same column
when moving from the workspace to the hotseat.) But this doesn't always
work, as in the attached bug where the hotseat doesn't get focus at all
when the All Apps button is an obvious candidate for it.

By adding a specialized column in the focus matrix for the All Apps
button, we ensure that moving down to the hotseat stays within the
original column when possible, while also allowing the focus to switch
to the All Apps button if appropriate. Furthermore, we take care to skip
over the All Apps column when necessary in order to maintain all
previous functionality.

Bug: 25590522
Change-Id: I5d6a8ee69de8834314c4689246fe7d54329b2eef