Add tests for AvatarPhotoController.

To facilitate adding tests, the class is modified to allow the
Activity to be mocked.

A small amount of clean-up is also performed, to remove AsyncTask
usages and remove some unnecessary code.

Bug: 221223842
Test: atest AvatarPhotoControllerTest
Change-Id: I70f3e8dcd371f48c780b29fada002dbd2f174c2d