Add flag for notif dismissal prototype

New flag: Settings.Global.SHOW_NEW_NOTIF_DISMISS
- Do not backup to other devices
- To be removed before release

Usage:
adb shell settings put global show_new_notif_dismiss [0|1]
- 0 to disable
- 1 to enable

Test: treehugger
Bug: 171817112
Change-Id: I8712a7f8a2bac6483afd52ede9e478f8b08d661d