[native] Update DisconnectedBar to display based on state.connectivity.connected

Summary:
Similar to D10469, but for native.

As mentioned in that diff, going to start by updating the DisconnectedBar on both native and web client to use network connectivity info instead of keyserver connectivity info.

Will then go ahead and rename eg useDisconnectedBarVisibilityHandler to make it clear that they're no longer connected to DisconnectedBar.

Depends on D10469

Test Plan:

1. Open native app on physical device
2. Kill local keyserver
3. Observe that nothing happens
4. Turn off WiFi
5. Observe DisconnectedBar

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat, ginsu

Subscribers: inka

Differential Revision: https://phab.comm.dev/D10475