Use original image instead of re-encoding as max quality JPEG

[orand]

MediaPicker was using UIImage.AsJPEG() to save the image picked from the camera roll. This re-encodes the image with maximum JPEG quality, resulting in an approximate doubling of the file size and loss of the original EXIF data.

This PR makes an exact copy of the original file from the asset library unless an edited version of the image exists, in which case it still uses UIImage.AsJPEG() since that is the only way to get the edited version of the image data.

I have tested and confirmed that this fix preserves the original file size and EXIF data for unedited images.

[orand]

Ooops! Please do not merge this PR yet! While it fixes taking pictures from the camera roll, it breaks taking pictures directly from the camera, which I wasn't able to test in the simulator. I'll be back with a fix.

[orand]

This was quite a bit more work than I expected, but this is now working correctly based on extensive testing on a real device. In addition to the original image file fix described above, I added better support for picking an image directly from the camera which previously always saved with maximum compression by calling AsJPEG() and tossed the original EXIF data. Now it adds the original EXIF data to the saved image and uses the compression quality you optionally provide with StoreCameraMediaOptions.JpegCompressionQuality which defaults to the compression quality used by the Camera app which results in a quality of 96 according to ImageMagick. This results in files that are 2x to 3x smaller than 100% quality.

[orand]

Updated the PR to add support for Unified API