Make bounds and scale of gif network images respond to device and styles

Summary:
@vjeux and I were discussing this in irc an discovered that network gif images did not respond how they should to width, height or flex properties. Along the way I also noticed that the scale was not changing depending on the device. This PR fixes that, so now you can do `flex: 1` on a gif image to have it stretch to the whole screen. [Minimum reproducible example here](https://gist.github.com/brentvatne/f745377b0789162a28df) - try this without and then with the changes of this PR to see.
Closes https://github.com/facebook/react-native/pull/353
Github Author: Brent Vatne <brent.vatne@madriska.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Image/RCTNetworkImageView.m

@@ -62,10 +62,7 @@
       _downloadToken = [_imageDownloader downloadDataForURL:imageURL block:^(NSData *data, NSError *error) {
         if (data) {
           CAKeyframeAnimation *animation = RCTGIFImageWithData(data);
-          CGImageRef firstFrame = (__bridge CGImageRef)animation.values.firstObject;
-          self.layer.bounds = CGRectMake(0, 0, CGImageGetWidth(firstFrame), CGImageGetHeight(firstFrame));
           self.layer.contentsScale = 1.0;
-          self.layer.contentsGravity = kCAGravityResizeAspect;
           self.layer.minificationFilter = kCAFilterLinear;
           self.layer.magnificationFilter = kCAFilterLinear;
           [self.layer addAnimation:animation forKey:@"contents"];