Fix Warning Invalid key color to ListItem (#299)
Fix Warning Invalid key color to ListItem (#299)

From the issues #250, #270, and from what I myself encountered after started using the Drawer, this warning was always shown in the app:  Warning: Failed prop type: Invalid props.leftElement key `color` supplied to `ListItem`.

As @aburai suggested in #250, leftElement and rightElement, not being set to `<View />`, they should be `PropTypes.style`, not `ViewPropTypes.style`.

Full warning text: 
```
Warning: Failed prop type: Invalid props.leftElement key `color` supplied to `ListItem`.
Bad object: {
  "color": "#2196f3"
}
Valid keys: [
  "display",
  "width",
  "height",
  "start",
  "end",
  "top",
  "left",
  "right",
  "bottom",
  "minWidth",
  "maxWidth",
  "minHeight",
  "maxHeight",
  "margin",
  "marginVertical",
  "marginHorizontal",
  "marginTop",
  "marginBottom",
  "marginLeft",
  "marginRight",
  "marginStart",
  "marginEnd",
  "padding",
  "paddingVertical",
  "paddingHorizontal",
  "paddingTop",
  "paddingBottom",
  "paddingLeft",
  "paddingRight",
  "paddingStart",
  "paddingEnd",
  "borderWidth",
  "borderTopWidth",
  "borderStartWidth",
  "borderEndWidth",
  "borderRightWidth",
  "borderBottomWidth",
  "borderLeftWidth",
  "position",
  "flexDirection",
  "flexWrap",
  "justifyContent",
  "alignItems",
  "alignSelf",
  "alignContent",
  "overflow",
  "flex",
  "flexGrow",
  "flexShrink",
  "flexBasis",
  "aspectRatio",
  "zIndex",
  "direction",
  "shadowColor",
  "shadowOffset",
  "shadowOpacity",
  "shadowRadius",
  "transform",
  "transformMatrix",
  "decomposedMatrix",
  "scaleX",
  "scaleY",
  "rotation",
  "translateX",
  "translateY",
  "backfaceVisibility",
  "backgroundColor",
  "borderColor",
  "borderTopColor",
  "borderRightColor",
  "borderBottomColor",
  "borderLeftColor",
  "borderStartColor",
  "borderEndColor",
  "borderRadius",
  "borderTopLeftRadius",
  "borderTopRightRadius",
  "borderTopStartRadius",
  "borderTopEndRadius",
  "borderBottomLeftRadius",
  "borderBottomRightRadius",
  "borderBottomStartRadius",
  "borderBottomEndRadius",
  "borderStyle",
  "opacity",
  "elevation"
]
    in ListItem (at Section.react.js:102)
    in Section (at AppDrawer.js:33)
    in RCTView (at View.js:71)
    in View (at ScrollView.js:717)
    in RCTScrollView (at ScrollView.js:811)
    in ScrollView (at Drawer.react.js:43)
    in RCTView (at View.js:71)
    in View (at index.js:14)
    in Container (at Drawer.react.js:42)
    in Drawer (at AppDrawer.js:14)
    in AppDrawer (at Main.js:30)
    in contentComponent (at DrawerSidebar.js:86)
    in RCTView (at View.js:71)
    in View (at DrawerSidebar.js:85)
    in DrawerSidebar (at withCachedChildNavigation.js:44)
    in withCachedChildNavigation(DrawerSidebar) (at DrawerView.js:112)
    in RCTView (at View.js:71)
    in View (at DrawerLayoutAndroid.android.js:176)
    in AndroidDrawerLayout (at DrawerLayoutAndroid.android.js:200)
    in DrawerLayoutAndroid (at DrawerView.js:134)
    in DrawerView (at DrawerNavigator.js:88)
    in Unknown (at createNavigator.js:13)
    in Navigator (at createNavigationContainer.js:214)
    in NavigationContainer (at Main.js:38)
    in ThemeProvider (at Main.js:37)
    in Main (at App.js:10)
    in RCTView (at View.js:71)
    in View (at App.js:8)
    in App (at registerRootComponent.js:35)
    in RootErrorBoundary (at registerRootComponent.js:34)
    in ExpoRootComponent (at renderApplication.js:35)
    in RCTView (at View.js:71)
    in View (at AppContainer.js:102)
    in RCTView (at View.js:71)
    in View (at AppContainer.js:122)
    in AppContainer (at renderApplication.js:34)
- node_modules\react-native\Libraries\ReactNative\YellowBox.js:71:16 in error
- node_modules\fbjs\lib\warning.js:33:20 in printWarning
- ... 32 more stack frames from framework internals
```
AppDrawer.js being the file Where the Drawer element is implemented.
xotahal/react-native-material-ui,xvonabur/react-native-material-ui