PixelRatio.pixel()

Summary:
This implements #5073. It adds a static method `PixelRatio.pixel()` which returns the smallest drawable line width, primarily for use in styles.

It also updates the example apps to use the new function.
Closes facebook/react-native#5076

Reviewed By: svcscm

Differential Revision: D2799849

Pulled By: nicklockwood

fb-gh-sync-id: b83a77790601fe882affbf65531114e7c5cf4bdf

react-packager/src/Resolver/__tests__/Resolver-test.js

@@ -10,6 +10,7 @@
 
 jest.dontMock('../')
   .dontMock('underscore')
+  .dontMock('PixelRatio')
   .dontMock('../../DependencyResolver/lib/extractRequires')
   .dontMock('../../DependencyResolver/lib/replacePatterns');