Fix libcore javadoc errors.

Random's javadoc was pretty random, and Pattern was missing some escaping
in code samples.

Also work round droiddoc bugs that were messing up Formatter and classes
that inherited documentation from Object.

Bug: http://code.google.com/p/android/issues/detail?id=13264
Bug: 3318601
Change-Id: Iddb0c807398840191ee003bc1644d611aef4d61d