health: only display system notifications for high severity warnings,…

[agottardo]

Updates tailscale/tailscale#4136

This PR brings the Android health system in line with recent macOS/iOS changes. Only high severity notifications will now trigger a system notification; meanwhile all notifications are now displayed in the app home screen, like we do on iOS. The "warming-up" Warnable is observed to prevent spurious notifications from appearing while the app has just launched.

Screenshot is only meant to be indicative of what a low-severity notification looks like, we don't actually show notifications when running an unstable build on Android.