Known exchange detection improvements - fix crash

Most important changes:

• Fix crash when wrong account name is pasted in input after valid exchange name

  reproduction was:

  1. Go on Transfer screen
  2. type mxchive or any other known exchange
  3. press space
  4. press backspace (so space will be removed)
  5. AssertionError: Shouldn't use <class 'clive.__private.ui.widgets.known_exchange_handler.KnownExchangeHandler'> with required memo input!

• Post KnownExchangeGone when going from valid to invalid account name (fixes missing update when changing state from "known exchange detected" to "invalid account name")