Rename useDisconnectedBarVisibilityHandler to useKeyserverReachabilityHandler

Summary:
Part of https://linear.app/comm/issue/ENG-5796/update-usedisconnectedbarvisibilityhandler

Basically we want to maintain the functionality that determines whether a keyserver is reachable, but we want to rename things so they're no disconnected bar sepcific (which will now display or not display based on general network connectivity).

Will need to make some Redux changes as well.

Depends on D10811

Test Plan:
NA, flow

Manually killed/started local keyserver a few times and ensured that Redux state updated as expected.

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10812