Put nested scroll logic in a separate modifier

- put the pointer input observer in a separate modifier so it does block
  rest of the logic
- only register the pointer input observer in non edit mode

Test: verified widget selection and vertical scrolling are both working
Fix: 356195210
Flag: com.android.systemui.communal_hub
Flag: com.android.systemui.hubmode_fullscreen_vertical_swipe
Change-Id: Ie434895310013263f1a04b0529b0e2a74e7fd3de