Add support for elevation in DrawerLayoutAndroid

Summary:It didn't work for a few reason. First, the drawer view NEEDS to have a background color or no shadow will ever render. Second, we need to use the `setDrawerElevation` method instead of `setElevation` for DrawerLayout. Finally we need to actually pass the style value (maybe we could just pass elevation but I don't really think it can cause any issues) down to the native component as it is not the case at the moment.

I also added a default style to elevation of 16 which is the standard for material design according to https://www.google.com/design/spec/patterns/navigation-drawer.html#navigation-drawer-specs. I could also default it to 0 so it keeps the same appearance as before but I think it looks better this way.

Closes #6022
**Test plan**
Tested using the DrawerLayout in the UIExplorer app.

Before, elevation 0
<img width="420" alt="screen shot 2016-02-23 at 1 55 42 am" src="https://cloud.githubusercontent.com/assets/2677334/13244000/008afdb2-d9d1-11e5-95b8-9c345ea0ea8d.png">

After, elevation
Closes https://github.com/facebook/react-native/pull/6100

Reviewed By: bestander

Differential Revision: D3012242

Pulled By: lexs

fb-gh-sync-id: 4967d7ec920f0229d823032ba95c8a3cace329c6
shipit-source-id: 4967d7ec920f0229d823032ba95c8a3cace329c6

Libraries/Components/DrawerAndroid/DrawerLayoutAndroid.android.js

@@ -156,7 +156,7 @@ var DrawerLayoutAndroid = React.createClass({
         drawerWidth={this.props.drawerWidth}
         drawerPosition={this.props.drawerPosition}
         drawerLockMode={this.props.drawerLockMode}
-        style={styles.base}
+        style={[styles.base, this.props.style]}
         onDrawerSlide={this._onDrawerSlide}
         onDrawerOpen={this._onDrawerOpen}
         onDrawerClose={this._onDrawerClose}
@@ -218,6 +218,7 @@ var DrawerLayoutAndroid = React.createClass({
 var styles = StyleSheet.create({
   base: {
     flex: 1,
+    elevation: 16,
   },
   mainSubview: {
     position: 'absolute',
@@ -230,6 +231,7 @@ var styles = StyleSheet.create({
     position: 'absolute',
     top: 0,
     bottom: 0,
+    backgroundColor: 'white',
   },
 });