Move NotificationAssistant flags into SystemUi namespace.

Create SystemUi DeviceConfig namespace and add a class to store all its
flags in. These flags will be shared between the SystemUi package and
other Notification packages - like ExtServices and
NotificationManagerService.

Performing experiments across different packages is much more involved
than performed single-package experiments. Therefore, we put flags
related to Notifications into the System UI experiment flag package.

Bug: 120792826
Test: atest AssistantSettingsTest
Change-Id: I83992291e660ab092bed8d22d1abaaa2bd9d6f9a