[native] introduce children prop to FullScreenViewModal

Summary:
Introduce children prop to FullScreenViewModal. This also factors out the rendering of Multimedia from FullScreenModal and instead we pass Multimedia in ImageModal This is step 3 in the list below

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

Depends on D9361

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

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9359