Enable system notifications

[AtlasLinden]

Allow SL notifications/pings to appear as an OS notification.

Initially let's start with direct messages, since chats are usually higher priority than Nearby chat or Group messages.

Test Plan

TBD

[akleshchev]

Is there a specific notification that should result in an OS notification?

Would be nice to have a list of notifications that should be covered, but we need at least one example to get started.

Also this probably should have either a global 'allow OS notifications' toggle or something on a per-notification basis like with 'ignore' setting.

[AtlasLinden]

One example to get started: Any chat notifications Will ask if product has any input on what else should be enabled here.