Replace Stetho with Flipper

Stetho is not deprecated yet, but could be soon: https://github.com/facebook/flipper/blob/master/docs/stetho.md

Flipper offers better debugging tools and integration seems as easy as Stetho. There is a random crash happening regularly when Stetho is attached to Chrome which is very annoying when you test the app.

Note: we should do this in FluxC as well.

  java.lang.NullPointerException: Attempt to read from field 'java.lang.Object com.facebook.stetho.inspector.elements.ElementInfo.parentElement' on a null object reference
      at com.facebook.stetho.inspector.elements.ShadowDocument$Update.removeGarbageSubTree(ShadowDocument.java:378)
      at com.facebook.stetho.inspector.elements.ShadowDocument$Update.removeGarbageSubTree(ShadowDocument.java:386)
      at com.facebook.stetho.inspector.elements.ShadowDocument$Update.removeGarbageSubTree(ShadowDocument.java:386)
      at com.facebook.stetho.inspector.elements.ShadowDocument$Update.removeGarbageSubTree(ShadowDocument.java:386)
      at com.facebook.stetho.inspector.elements.ShadowDocument$Update.removeGarbageSubTree(ShadowDocument.java:386)
      at com.facebook.stetho.inspector.elements.ShadowDocument$Update.removeGarbageSubTree(ShadowDocument.java:386)
      at com.facebook.stetho.inspector.elements.ShadowDocument$Update.commit(ShadowDocument.java:360)
      at com.facebook.stetho.inspector.elements.Document.applyDocumentUpdate(Document.java:494)
      at com.facebook.stetho.inspector.elements.Document.updateTree(Document.java:346)
      at com.facebook.stetho.inspector.elements.Document.access$1200(Document.java:31)
      at com.facebook.stetho.inspector.elements.Document$ProviderListener.onPossiblyChanged(Document.java:720)
      at com.facebook.stetho.inspector.elements.android.AndroidDocumentProvider$1.run(AndroidDocumentProvider.java:71)
      at android.os.Handler.handleCallback(Handler.java:739)

An issue has been open for a long time: https://github.com/facebook/stetho/issues/652

Start by reading the referenced Flipper documentation and locating the existing Stetho integration in WordPress for Android and FluxC. Compare the integration requirements for both projects; done means Stetho is replaced in both and the reported crash when debugging through Chrome is no longer observed.