Simplify WebViewProviderInfo - move its logic into WebViewUpdateService.

The WebViewProviderInfo should now be ready to be added as an API to be
fetched from XTS tests (to avoid using reflection).

Move the logic for validation, signature checking and package info
fetching out of WebViewProviderInfo so that we can mock the coupling
between that logic and the system (e.g. the package manager).

Note: with this patch we stop caching valid webview packages in the
update service (we would still refetch them anyway when anything
important happened).

Bug: 27635535
Bug: 27736084

Change-Id: Ia455202d2fd5bc4e03dce0fd917d262bf942d1a3