Show mediator connection(s) under app Settings

[cvarjao]

• Show Mediator contact hidden by default, shows up when developer mode is enabled
• Show list of mediator connections
• A user may tap and go to the "contact details" screen for a specific mediator

[wadeking98]

Resolved by: https://github.com/hyperledger/aries-mobile-agent-react-native/pull/642