[keyserver] Update updateUserAvatar to construct ClientImageAvatar

Summary:
Update updateUserAvatar to return properly constructed ClientImageAvatar if the request was of type image.

Depends on D7404

Test Plan:

1. Set image avatar for user
2. Observe that UPDATE_USER_AVATAR_SUCCESS payload includes correctly constructed ClientImageAvatar
3. Observe that UserAvatar component looks correct throughout native

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7406