[Android] Replace a synchronous callsite of GetAccounts().

Two misc changes from http://crrev.com/1380103004
Use a static JNI call from java to C++.
Make the GetAccounts method asynchronous.

TBR=rogerta@chromium.org

BUG=517697

Review URL: https://codereview.chromium.org/1405353002

Cr-Commit-Position: refs/heads/master@{#354517}
5 files changed