Add predictions row to taskbar all apps.

- We need to listen to DeviceProfile changes in case the number of
  columns changes in the grid. I made an interface/mixin separate from
  ActivityContext to avoid polutting the latter with too many things. I
  also applied this change to existing taskbar A-Z grid.
- Added all apps visited count to onboarding preferences for only
  showing "All Apps" label in place of divider 20 times. Label is also
  tracked on taskbar side and should be kept in sync.

Test: Manual
Fix: 216843395
Bug: 174174514
Change-Id: I97aa91397c334123626caf18251f19e17c7104fb