[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=
In conclusion, current implementation is
1) XWalkView will pause everything after activity paused and resume them back after activity resumed.
2) Embedder can additionally call these apis to pause/resume things any time they want.
3) It's technically possible to overwrite the behavior since it's a public method of XWalkView, but it's not in Embedding API document yet.