[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=
`onHide/Show` is doing the similar thing as `pause/resumeTimers`, ideally, we should also leave the choice to embedders.
However, here is more like a default impl XWalkView provided. I will propose to expose this API in next Embedding API iteration.