Summary: This PR includes a working interface to Android's `elevation` property implemented as an RN style. Elevation is the only (easy) [platform-supported way to create shadows](http://developer.android.com/training/material/shadows-clipping.html). For it to work note that you must be running on Android 5.0+, and add `elevation` to a view with a `backgroundColor` set. These are platform limitations.
This PR is not intended to be merged in its current state, but rather to inform the discussion from #2768. At a minimum, before merging we would need to add the elevation style to the docs and rebase this to master (**EDIT** I have now rebased on master because from v0.14.2 too many commits were being pulled in -- haven't tested it since the rebase though). Additionally, it might be good to add tests, although I couldn't find any for the Android code. I'm happy to get that done if this feature gets signed off by the React Native team.
Finally, as I argued in #2768 I think that `elevation` is a useful abstraction ov
Closes https://github.com/facebook/react-native/pull/4180
Reviewed By: svcscm
Differential Revision: D2684746
Pulled By: mkonicek
fb-gh-sync-id: 825f3ccd20c4b0eea9d11b5f0e3a6b018b7e4378
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