Untie WebSocket implementation from Android

[om26er]

Fixes https://github.com/crossbario/autobahn-java/issues/419

WebSocket

• Abstract background thread <==> UI thread communication to remove the hard dependency on Android specific imports.
• Allows to use WebSocket impl on non-android platforms, making unit testing much simpler

  Cleanups

• Add missing license headers.

From a developer's perspective, nothing changes, all the changes are internal.

[oberstet]

fwiw, as mentioned on slack, this is awesome, and I am +1 on merging (can't really comment on details .. as I haven't enough working knowledge of the android/netty/java build script bits involved)