BoundedRational.java cleanup

Bug: 21469726
Bug: 23080519

Reformat to 100 columns.  Reformat simple if-statements and rename
variables for better consistency with coding conventions.

Switch many comments to javadoc style.

Consistently use signum() to compare to zero.  It's more concise and
probably slightly more efficient.

Change the asBigInteger implementation to use a single
divideAndRemainder call instead of relying on gcd. Add tests.

Eliminate redundant test in maybeReduce().

Change-Id: I4c275494e076612d09a05bc317c9972008619cda
(cherry picked from commit a4511f34)