Store the WebView's package info.

When loading the WebView, save the PackageInfo for the package so that
it can be retrieved later to access the package name, version data, etc.
This will be used to detect WebView downgrades that may affect an app's
saved WebView-related data.

Bug: 16724751
Change-Id: I05ce469308cc87f79c73206327f43b76b1bab222