Currently (as of MoPub Android SDK 4.9.0) MoPubStreamAdPlacer is 2 degrees away from the MoPubNative class, making it difficult to pass
localExtras. That is, a MoPubStreamAdPlacer wraps NativeAdSource which wraps MoPubNative.
It would be great to add a public
setLocalExtras() to MoPubStreamAdPlacer, which would pass it down to MoPubNative, so that developers would not need to copy classes or do other ugly workarounds to set the
localExtras field on native ads.