[native] introduce FullScreenViewModal

Summary:
To view a full screen avatar for user profiles we are going to want to reuse parts of the ImageModal since it has all the same zooming/panning/gesture functionality we need. However, right now there are parts of ImageModal that are specific to viewing a multimedia message. We need to factor those parts out, so I created FullScreenViewModal which will be a generic component that can be used in both cases for image multimedia messages and user avatars.

Outlined below are the steps I will take in this stack (each point here will be it's own diff):

1. Introduce FullScreenViewModal file and move everything from the ImageModal file into here, and change what is absolutely necessary (the imports)
2. Correct the naming inside of FullScreenViewModal
3. Lift out rendering of <Multimedia /> in favor for a generic content child
4. Lift save content / copy content functionality out of FullScreenViewModal and make those optional props
5. Lift the imageDimensions function out of FullScreenViewModal and introduce a required generic contentDimesions prop

Test Plan: Confirmed that there were no regressions with the viewing an image multimedia message experience

Reviewers: atul, inka, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9360