AW docs: signing key install error to troubleshooting

Only changes docs.

This adds `adb install`'s signature mismatch error to the
troubleshooting. I realized this may happen if a Googler has installed
an official build of Chrome Debug (signed with dev-keys) and then tries
to install a locally compiled SystemWebView.apk with the same package
name, but without `use_signing_keys = true` (which is not generally
required for the quick-start).

This may be uncommon, but it's certainly possible and there's no reason
not to document it.

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I4c4de1209e0222e226445a907ed9bf6ad528a222
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1676683
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672259}
1 file changed