Changed test to wait for paint complete before focus request.

 Bug 5688191
 WebViewTest.testSetDownloadListener was requesting focus with
 View.FOCUS_DOWN immediately after loading the page. The view
 size did not get updated until the paint was complete,
 which can happen any time later on the WebViewCore thread.
 Changed the test to call waitForIdleSync prior to requestFocus
 to ensure that the FOCUS_DOWN correctly handles the faked
 down arrow event.

Change-Id: I8f7047e8bec238850534e2fafa2befe4956b6cf0