Allow taskbar and taskbar all apps window to receive split touches

Adding feature to allow UI interaction during Drag and drop.
To allow user to interact with taskbar UI during D&D, the window
should support FLAG_SPLIT_TOUCH to receive touch events while user
using D&D.
DD: go/global_drag_and_drop

Test: manual
Bug: 220109830
Change-Id: Icd549aa272c140d865d89eeb9f5619b5ab13c015