Update Android junit docs to make it clear how to specify out path.

Previous command-line specified --release which only worked if you
build under out/Release. New instructions make it clear how to
specify a custom one - uses my_build which GN docs use as an
example too.

BUG=none
[email protected]

Review-Url: https://codereview.chromium.org/2692273004
Cr-Commit-Position: refs/heads/master@{#450464}
1 file changed