Fix android.webkit.cts.WebViewTest#testFindNext seldom fail

In this test case,  the assertion of ScrollY value after the first findNext()
operation could fail when the findNext() is invoked before findAll() is completed.
In order to use findNext() method of WebView to find the text on page we should
make sure that the findNext() operation is valid. That means the findAll()
operation should be completed before findNext() because valid findNext() is
must based on the findAll() result "matches".
We suggest that this test case should use setFindListener() method of WebView
to insure the findNext() operation is valid. Just like the test case testFindAll.
Signed-off-by: Junjie.Hu <junjie.hu@mediatek.com>

Change-Id: I4f5d3e20c4b359b7f84ed0a22623582aaaede06b