JNI: Add test rule and sample test for mockable natives

Bug: 898261
Change-Id: Ie1e5c545bdf23adc232bee0c34abde3413c4f943
Reviewed-on: https://chromium-review.googlesource.com/c/1348632
Commit-Queue: Aiden Benner <abenner@google.com>
Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614563}
5 files changed