Summary:
This is a followup to PR #3850 but now separates min/max track images into different properties.
Closes#4476
Add examples for `minimumTrackTintColor`, `maximumTrackTintColor`, `minimumTrackImage`, `maximumTrackImage` to UIExplorer.
Closes https://github.com/facebook/react-native/pull/4586
Reviewed By: svcscm
Differential Revision: D2779193
Pulled By: nicklockwood
fb-gh-sync-id: 0510a0f496816baacdd0d4be0f3cd3a63a5a9865
Christopher Dro