sandwich: Allow saving cache directory
sandwich: Allow saving cache directory

Adds an option --save-cache to run_sandwich.py to pull the cache directory from
the device after all page loads completed, preserfing modification times.

BUG=582080

Review URL: https://codereview.chromium.org/1651193002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#372931}

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