Change ValidJwtTokenAvailable Dbus signal into generic JwtTokenStateChange signal

Acceptance criteria:

• ValidJwtTokenAvailable is changed into JwtTokenStateChange
• The signal is emitted when any of these conditions are true:
  • A new token has been obtained
  • The server has returned Not Authorized when calling the auth_requests endpoint
• The last one should be emitted even if the client was already unauthorized (important to allow clients to quickly determine that the client is unauthorized after calling FetchJwtToken)
• Token is broadcasted in the signal, if available
• Acceptance test from MEN-4036 is adjusted to match
• mender-shell is adjusted to match
  • waits for a valid authtoken on startup
  • closes websocket connection when device is unauth
  • goes back to waiting for a valid token