Discard changes dailog (1/2)

The dialog pops up when users make changes to the preivew but yet apply
the changes and try to leave the screen.

Test: Manually tested
Fixes: 376092973
Flag: com.android.systemui.shared.new_customization_picker_ui
Change-Id: I3d93183edda086a301a912acee17c9a19467cae3