Dispatch insets from overlay drag layer.

This simplifies insets handling for All Apps and EDU, and also fixes
some edge cases with All Apps popup menus. KeyboardQuickSwitchView now
needs to ignore insets since it has a hardcoded top margin,
keyboard_quick_switch_margin_top.

Fix: 276804708
Test: All Apps, EDU, and Quick Switch all have proper insets handling.
Change-Id: I93237a057b4f9ae62f71a70c59a58f3c8401c102