Fix loading of null webview if there is no webview package.

In the change in go/ag/672863 we throw an AndroidRuntimeException
instead of a PackageManager.NameNotFoundException when no webview
package exists. This should break devices not using webview since
the NameNotFoundException is used to detect webview not being present
(so that the null webview can be loaded instead). In this patch we
create a new type of exception and look for that one instead when we
want to determine whether the device should or should not have webview
installed.

Change-Id: Ia75dec718d7a5b2c3517671c54be3950badb8bba