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Optimizes the Android Skia Gold code in the following ways:

1. Adds an "Initialize" step and shuffles arguments around so that
   goldctl only has to initialize the working directory once
   instead of every time a comparison is performed, saving ~250ms
   per comparison.
2. Because #1 causes each session to only be valid for a single
   instance/corpus/keys_file combination, adds
   SkiaGoldSessionManager to lazily create sessions as necessary.
3. Short circuits the "Authenticate" step if the session is already
   authenticated, saving ~40ms per comparison.

Change-Id: I72677ec8b0a0a3757ce198e8b8e1d34be7b74257
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148153Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759703}