Prevents 1 to 1 conversations of the same id on different accounts
(local) from having the same avatar after switching account.

Change-Id: Id30af117b2da8bd35d64b54f2e02cbbed0710848
Gitlab: #466

Implements a leaner avatar caching system. The avatar component
listens for uid filtering its id, which may be:
- conversation id
- account id
- contact uri

In response to the uid change, a the image source is updated with
a new image url invoking a fresh QQuickImageProvider query. With
this design, only the avatarregistry's uid mapping needs to be
updated when profiles are changed, and no longer should specific
avatar components receive manual source updates.

Gitlab: #466
Change-Id: Ie5313f5c187a0977ca51b890dd92187480a42537