[TIMOB-15363] iOS7: TiRootViewController should use launch image from Info.plist if specified

In iOS 7 you can specify iOS version specific launch images in the Info.plist file like this:

<key>UILaunchImages~ipad</key>
<array>
        <dict>
                <key>UILaunchImageName</key>
                <string>Default7-Portrait</string>
                <key>UILaunchImageMinimumOSVersion</key>
                <string>7.0</string>
                <key>UILaunchImageOrientation</key>
                <string>Portrait</string>
                <key>UILaunchImageSize</key>
                <string>{768, 1024}</string>
        </dict>
</array>

This works fine if specified in the tiapp.xml as iOS display the correct image. Unfortunately after the launch image is displayed and before the first app view opens, the TiRootViewController displays the launch image again, but this time only the standard files like Default.png or Default@2x.png are used. The TiRootViewController should also use the launch images specified in the Info.plist file (if one is specified for the current iOS version) for a consistent startup behaviour.