Ensure at least one default-available webview package is a non-fallback

Parts of the WebViewUpdateService code assume that there exists at
least one webview package which is available by default and is not a
fallback package. With this CL we enforce this assumption by crashing
when parsing the webview packages config file if it doesn't hold.

Bug: 28014401
Change-Id: Ib7de1f596e31bae65542a6883a9f3af54337bf1b