Reorder the "Prerequisite" for android emulator doc.

Also add the marshmallow config & builder to the table,
and the doc for `--debug-tags` option

Change-Id: Ife316d632342025899faf4b921f83b9d009ed32d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523358


Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#824922}