[Android] Add onStart and onStop to XWalkExtensionClient

Added the onStart() and onStop() method in XWalkExtensionClient,
and also added these two functions in the related classes to
make it work for XWalkExtensionClient. For now extension developer
can override the onStart() and onStop() when the activity's
onStart() and onStop() get triggered.

BUG=XWALK-1708
4-space indent.