Add more checks for invalid states when reading webview packages config.

There should be at least one available-by-default package (otherwise the
user would have to explicitly choose webview implementation before using
webview). Furthermore a fallback package should always be available by
default (otherwise it would have to be chosen by the user to be used
after being enabled).

Bug: 28014401
Change-Id: I90c300898a5be4219317d65f15878e1d421193d7