Effective Java (Joshua Bloch) Item 17 says :
"Design and Document or inheritance or else prohibit it"
However, just a cursory glance through the Android APIs reveals that most of the API classes are non-final; which is OK if they are also documented for inheritance (View of Activity, for example). But there are also several non-final classes, but the documentation makes no mention about the inheritability of these classes. Just some arbitrary examples to illustrate my point:
WifiManager, NotificationManager ...)UriMatcher.Camera.Openness and extensibility being the philosophy of Android, is the convention reversed here? Meaning, could one assume that all of the Android API classes are designed to be inherited (whether explicitly documented or otherwise); unless declared final?
Just my €0,02: Clean OO design by the book is one thing, making things work for all possible use cases in practice is another. The principles of clean OO design sometimes are somewhat of academic nature. - And maybe a little bit of black and white.
Think for instance about who uses the Android API provided by google: It's not only app developers but also device manufacturers who need to specialize general APIs for their devices.
So, for me, SW design is neither black nor white in most cases; the shades of grey are important.
And finally: In practice I have seldom (read: never) seen problems caused by "carelessly" omitted final keywords (on classes), while unreflected overuse of final (often caused by thoughts like "my code is sooo [great | ugly], no one will actually ever want to modify it through inheritance") can be quite a pain.
"I know that I know nothing" seems to fit here: It is presumptuous to claim that one knows all the crazy, ingenious, creative, smart,... ideas of others for how one's code may be used in the future beforehand.