Update NullWebView interface to match real WebView.

The real WebView is created via a static factory method now; add the
corresponding method to NullWebViewFactoryProvider so they can be
created uniformly.

Bug: 37645436
Test: cts-tradefed run commandAndExit cts -m CtsWidgetTestCases -t android.widget.cts.TextViewTest
Change-Id: Ic8e1dd84e3b78d3a0173ff13e5e849c224a3cd78