[Android] Multi-frame support for Java extensions

[1] Exposed instanceID up to Java side when it receives message which can be used to identify the target instance       when post message to JavaScript side.
[2] Add a new interface "broadcastMessage" to meet the requirement that Java side may need to broadcast message to all exiting instances
[3] Add unit test for the new interfaces against xwalk_core_shell_apk
[4] This PR has also been tested against xwalk_runtime_lib_apk, but there is no evidence here. Please refer https://github.com/DonnaWuDongxia/crosswalk/tree/multi-frame-with-runtime-test for detail.

BUG=https://github.com/crosswalk-project/crosswalk/issues/705