Don't allocate a CADisplayLink for static images

Summary:
@public
The mass majority of RCTUIImageViewAnimated uses are actually for static images. As such, we don't need to create a CADisplayLink.

Reviewed By: shergin

Differential Revision: D16945038

fbshipit-source-id: a7cb63000987d1ea7a8a9b4d596e1e474709d2ac

Libraries/Image/RCTUIImageViewAnimated.m

@@ -146,6 +146,12 @@ - (NSOperationQueue *)fetchQueue
 
 - (CADisplayLink *)displayLink
 {
+  // We only need a displayLink in the case of animated images, so short-circuit this code and don't create one for most of the use cases.
+  // Since this class is used for all RCTImageView's, this is especially important.
+  if (!_animatedImage) {
+    return nil;
+  }
+
   if (!_displayLink) {
     _displayLink = [CADisplayLink displayLinkWithTarget:[RCTWeakProxy weakProxyWithTarget:self] selector:@selector(displayDidRefresh:)];
     NSString *runLoopMode = [NSProcessInfo processInfo].activeProcessorCount > 1 ? NSRunLoopCommonModes : NSDefaultRunLoopMode;