Fix WebViewTest.testGetOriginalUrl()

This test requires a callback to be made to
WebViewClient.shouldOverrideUrlLoading(), which happens on the UI thread. We
must therefore yield the UI thread while waiting for the load to complete.

This was broken by the addition of the @UiThreadTest annotation in
https://android-git.corp.google.com/g/#/c/109366.

Bug: 5327443
Change-Id: I3c7ccb3260b531c98ad62b187200440070e83593