Move current WebView provider from Settings.Secure to Settings.Global

The current WebView provider is not user-specific and should therefore
be stored as a Global rather than a Secure setting.

Also do some code cleaning including a fix in WebViewProviderInfo to
always fetch up-to-date information about whether a webview
implementation package is enabled.

Bug: 27142972
Change-Id: I4d4b8fca775e97980fb5c34313be6d82472e7d33