[Android] Add test case for OnJavascriptModalDialog.

This patch is to add test case for OnJavascriptModalDialog().
[Android] Add test case for OpenFileChooser.

This patch is to add test case for OpenFileChooser().
[Android] Add test case for OnFullscreenToggled.

This patch is to add test case for OnFullscreenToggled().
[Android] Add test case for shouldOverrideKeyEvent.

This patch is to add test case for shouldOverrideKeyEvent().
Load the data, simulate the key action, XWalkUiClient will
receive shouldOverrideKeyEvent.
[Android] Add test cases for onPageLoadStarted.

This patch is to add test cases for onPageLoadStarted().
Test this interface with local url, invalid url and webserver.
[Android] Add test case for onUnhandledKeyEvent.

This patch is to add test case for onUnhandledKeyEvent().
[Android] Add test cases for onPageLoadStopped.

This patch is to add test cases for onPageLoadStopped().
Test this interface with verify the url and load status.
Merge pull request #2308 from wuhengzhi/xwalk_ui_client_test

[Android] Add test cases for the UI related APIs.