[native] Unlink files as necessary in useUploadSelectedMedia

Summary:
expo-image-picker will always copy the contents of image into cache and provide a URL to cached image:

As a result we'll ALWAYS need to unlink the selection.uri. We also clean up processedMedia.uploadURI if necessary.

Test Plan:
Tested small images which weren't processed and large images which were processed. Logged the contents of selection, processedMedia, and urisToBeDisposed. When I commented out the unlink line (159), I was able to open all of the images in Finder. When I uncommented the unlink line, all of the images were inaccessible because they'd been removed:

Before:

After:

Tested Android by ensuring that files remained in caches when unlink was commented out and weren't in caches when unlink was uncommented:

Selected an image avatar and ensured contents were as before:

Selected an image avatar with unlink left out and observed that new files were added:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7704