[Android] Change the super class of XWalkRuntimeView to LinearLayout

Advertising extension, refer to
https://github.com/crosswalk-project/crosswalk-android-extensions/pull/25,
will insert the ad view, which cannot cover the HTML content. Currently, the
supper class of XWalkRuntimeView is FrameLayout, which isn't able to shrink
when a view is inserted. So it will be replaced by LinearLayout.