Efficient conversion to read-only java.util.List/Set/Map

[odenix]

Currently, converting to java.util.List/Set/Map requires a full copy.

Options:

1. Add ImmutableList/Set/Map.asJava() that returns a read-only java.util.List/Set/Map view
2. Change ImmutableList/Set/Map to extend java.util.List/Set/Map
   Currently impossible due to method name clashes, but upstream is considering to rename add -> adding, etc.

(1) gives a cleaner API and implementation, whereas (2) avoids the downsides of wrapping.