Fix for a crash when exiting an app:
Activity.onPause() -> MessageSender.stopSending sets mThis to null, then a last message gets sent and a crash in newMessage().
Fix this by
1) making the Message inner class static (so that we can create a new one without a reference to mThis)...
Simplify a bit the place where we got a crash once.
Simplify the way applications can get notifications when an effect finishes.
Now, instead of the 'DistortedEffects.(de)registerForNotifications()' 2 APIs, we call a single 'Effect.notifyWhenFinished()'.
Correct the EffectMessageSender.
Correct the Legal notice.
Rename all the classes that are not exported to application to 'Internal'
Move EffectMessageSender to the 'message' package (duh!)