Added a java wrapper for base::UnguessableToken.  Native code can
start with a native UnguessableToken, and convert it to and from a
java UnguessableToken.

The java implementation is Parcelable, so that it can be sent via
binder easily.

TEST=jni_unguessable_token_unittest
BUG=

Review-Url: https://codereview.chromium.org/2468353002
Cr-Commit-Position: refs/heads/master@{#431342}