Global bubble settings

Background: Bubble settings have the following levels
- Global (developer options flag)
- App-level
- Channel-level

Issue: Global settings do not propagate to the work profile user because we are
storing NOTIFICATION_BUBBLES in the Secure Settings table instead of the
Global Settings table. Settings are global XOR per-user.

This change
- Sets default value for NOTIFICATION_BUBBLES in global settings table.
- Adds Global.NOTIFICATIONS_BUBBLES to frameworks/base/api/test-current.txt
- Keeps old upgrade steps as no-op to keep db versioning stable.

Bug: 136034310
Test: RunSettingsRoboTests ROBOTEST_FILTER="Bubble"
Change-Id: I21d3dae75e2a4ab0fac10f3d7ea7eed16e0f5d6c