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This replaces the unsigned launchd script with a signed script inside
the ME2ME host bundle. This is needed so that we can make changes to
the script without having the user re-authorize the host in
System Preferences -> Security & Privacy. This also helps with
Catalina signing/notarization, because it is expected that Apple's
notarization service and/or Gatekeeper will reject an unsigned script.

Changes:
* org.chromium.chromoting.me2me.sh removed from the installer package.
* New script added to the host bundle (with very minor changes needed
  for it to work). New script is without ".sh" extension, making it
  language-agnostic (in future, we could convert to a binary).
* New script is signed by do_signing.sh - this is required in order to
  sign the overall host bundle.
* Launchd agent plist updated to point to the new script.
* Upgrade will preserve the old script and plist, so that users don't
  have to re-authorize. New installs will use the new script. This is
  achieved by backing up the old script+plist in preflight and
  restoring them in postflight.
* Updated ME2ME native-messaging host setup to point to the new script.
* Permission prompt updated with new name (instead of the ugly string
  org.chromium.chromoting.me2me.sh).
* Uninstaller removes the old script if it exists.

Bug: 996989
Change-Id: Id3d7a0a9f666178b6871458b96f0af8177beb301
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815977Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698701}