[lib] Rename updateDisconnectedBarActionType to updateKeyserverReachabilityActionType

Summary:
This is a simple rename that only affects clients.

Next diffs will be:

• native migration to replace connection.showDisconnectedBar with connection.reachable WHERE reachable will be initially set to !showDisconnectedBar
• Same as above, but for web.
• Modify keyserver to serve showDisconnectedBar for old clients and reachable for new clients.

Test Plan: This is a straightforward rename. The next diffs in the stack will need to be tested thoroughly.

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10879