[Android] React to activity state change within XWalkView

Previously, XWalkView relies on embedders to manage the
resource of XWalkView based on Activity lifecycle.

In this commit, implement XWalkView as ActivityStatusListener
and handle the lifecycle state change by itself.

BUG=
Since the default behavior of XWalkView is updated, it would be better to add javadoc comments for XWalkView class to describe such default behavior, and tell embedder to how to override it, what do you think?