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This refactors how we plumb args.platform through run_cts.py, and
provides clearer error output:

 * If your device is higher than the supported platform range, we
   recommend using the max supported platform
 * If your device is lower than the supported platform range, explain
   that we don't support this because WebView is not updatable
 * If we auto-select a platform, this writes an info log to explain what
   platform we chose (useful for debugging)

This renames --platform to --cts-release, since "--platform" was a bit
unclear, and improves the --help description to better explain how to
use this.

This also adds unit tests for the logic behind the actionable
exceptions.

Bug: 934170
Test: vpython android_webview/tools/run_cts_test.py
Change-Id: I4574ebf1a760c47e6ae09389ebb73b7ece1967d5
Reviewed-on: https://chromium-review.googlesource.com/c/1481076Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634579}