Mojo: Add a way to create thread-safe interfaces in Java

This change adds Interface.Manager.buildThreadSafeProxy(), which is
roughly analogous to mojo::ThreadSafeInterfacePtr<T>. Given that Java
does not have move-only semantics, the Proxy object that is passed is
unbound and a new object is returned.

Bug: None
Test: cheets_ContainerSmokeTest in Chrome OS
Change-Id: I6565f9e526e3fa8f8cb222cb8cd11e95bb57f7d3
Reviewed-on: https://chromium-review.googlesource.com/1147320
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Luis Hector Chavez <lhchavez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577429}
2 files changed