Add functionality for changing WebView provider.

Make it possible to change WebView provider (through a Developer
setting) and kill all apps using the old provider.
This includes checking the signatures of the WebView providers to make
sure they are valid.

Now that we can change WebView provider through a setting it is possible
to change provider while some provider is being updated. Because of this
we now keep track of which provider should be in use in
WebViewUpdateService to make sure we use the correct provider at all
times.

We now also read WebView package meta data (name, package name, and
signature) from a separate xml file.

Main bug: crbug.com/546185

Bug: 25338573

Change-Id: I660fd1a40a5388f6569a06a7f0d029e8ff65945a