[Android] Implement the interface for onScaleChanged.

This patch is to add the bridge code for onScaleChanged.
In xwalk_render_view_host_ext.cc, it receives the event about page
scale factor changed, transfers the factor to XWalkContentsClientBridge.
Merge pull request #2275 from wuhengzhi/onScaleChanged_bridge

[Android] Implement the interface for onScaleChanged.