locationsharing: add notification

Show notification when a peer is sharing its position on the non
current conversation.
This needed a lot of changes. Now we store shared position via
accountId + peer.
This also fix location icons and keep state when changing from
one account to another.

Change-Id: I8c1848890efa09f6e296e9da779a355167e4d3d4
GitLab: #888