move notification icon to sample project

This puts the lib one step closer to only being a jar!

refs #3680 https://dev.guardianproject.info/issues/3680
purge CacheWordActivityHandler, its a pointless wrapper of CacheWordHandler

Instead, apps should just instantiate CacheWordHandler and call
connectToService() and disconnectFromService() where appropriate.  That is
most likely going to be in an Activity's onResume() and onPause()
respectively.
move getApplicationContext() into getBlankServiceIntent()

Since CacheWordService.getBlankServiceIntent() requires a long-lived
Context, move the call to getApplicationContext() into the method itself to
guarantee that it gets the Application's Context.
purge built-in Notification, the app must now provide it

Previously, there was an elaborate, custom framework for customizing the
Notification that was hard-coded into CacheWord.  Now, the app must supply
a Notification, and that triggers CacheWordService to run as a foreground
Service.  Otherwise, CacheWordService will run as a background Service with
no Notification.

This removes the last remnants that require CacheWord to be an Android
Library Project.  Now it can be distributed as just a jar.  It can still be
used as a Library Project for those who like doing it that way.

fixes #3711 https://dev.guardianproject.info/issues/3711
fixes #3680 https://dev.guardianproject.info/issues/3680