Adds ENABLE_FLOATING_SEARCH_BOX flag for All Apps.

This defines how the All Apps screen should be laid out without
anchoring to the search bar at the top, as follows:
 - Header view aligns to the top instead of below search bar.
 - Same for A-Z list.
 - Scroller is aligned to the header view to receive the above
   adjustments automatically.
 - A-Z list is set above search bar to not peek from below.
 - Search bar is set to align parent bottom and translates up
   with the keyboard.
 - Button to disable work apps is raised above the search bar.

Bug: 213954333
Test: Manually with flag enabled/disabled, Always show keyboard
enabled/disabled, and work profile enabled/disabled.

Change-Id: If90bb39a890029fa7056367fe62bad0677f0b86e