This commit also merges new changes to checkstyle.xml and fixes new checkstyle
warnings related to Javadocs.

709d97aa321d5729988fd63b960bbece04cfba10 modified the -AskipDefs argument to the
Checker Framework (a regular expression) in a way that caused it to skip
checking all files.  This commit fixes the regular expression and the new
Checker Framework warnings.

This commit uses the Checkstyle ImportControl feature
(http://checkstyle.sourceforge.net/config_imports.html#ImportControl) to specify
the allowed imports for each package.  Specifying imports has several benefits:

- It makes some parts of the design more concrete.  For example, it shows that
  'stats' can depend on 'tags', but 'tags' shouldn't depend on 'stats' or
  'trace'.

- We can use it to restrict access to 'internal' packages.

- It can help us remove the dependency on Guava incrementally.  For example,
  this commit specifies that the API can only depend on
  com.google.common.io.BaseEncoding, which should help us avoid accidentally
  adding more uses of Guava to the API.

One downside is that the check seems to only apply to import statements, not
qualified class names.

This commit specifies imports positively, using "allow" XML elements.  If the
lists of imports are too difficult to maintain, especially for packages such as
exporters that have many imports, we could switch to using "disallow" elements
in some places.