ringaccount: factor peer authorization check

Previous implementation was duplicating authorization
checks between incoming call and message.
Add two methods to factorize the checks:
* foundPeerDevice will check the validity of the peer certificate chain
* onPeerMessage will check the validity of the peer certificate chain,
  perform authorization checks using the account trust store (async),
  and call the provided callback if the peer is authorized.

Change-Id: I80833a73c6842c9e84677fcaaef2a543745e005b