[native] improve factoring out redundant logic for saving an image user avatar

Summary:
This diff has two small improvements for factoring out the redundant logic for saving an image avatar. The first improvement makes the callback in useUploadSelectedMedia return a ImageAvatarDBContent instead of a string. The second improvement factors out the update image avatar logic into its own function

Depends on D7622

Test Plan: flow and editing image user avatar still has expected behavior

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7628