Expose the Site Engagement Service to Java.

This CL adds a Java-side interface to the Site Engagement Service, as
well as a basic integration test for the interface. This allows Java
code (particularly tests) to programatically adjust and retrieve the
score.

BUG=671100

Review-Url: https://codereview.chromium.org/2553013002
Cr-Commit-Position: refs/heads/master@{#437825}
9 files changed