[native] make handlePhotoCapture return mixed instead of void

Summary: Based on feedback I got in D7624 we should change the type of handlePhotoCapture so that we can make CameraModal even more permissive

Test Plan: flow and double checked to make sure that the camera modal still has the same expected behavior

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7676