Remove dependency between XWalkManifestReader and XWalkView.

XWalkManifestReader is in org.xwalk.runtime package, it's on higher
layer than XWalk core, and XWalkManifestReader should not be awared
by XWalk core. So remove the XWalkManifestReader reference from the
XWalkView class.

BUG=https://crosswalk-project.org/jira/browse/XWALK-149