[Android]Fix build error said: multiple definition of
'xwalk::XWalkRenderProcessObserver'.

The PR#1711 defined another xwalk::XWalkRenderProcessObserver class
which has the same class name in Android port, so exclude the
xwalk_render_process_observer_generic.cc(h) files in Android port.
Merge pull request #1743 from takethathe/android_build_fix

[Android] Fix build error said: multiple definition of 'xwalk::XWalkRenderProcessObserver'.