[Android] Make devtools server be singleton in application wide

Currently each XWalkView instance will create a XWalkDevToolsServer
instance, it doesn't make sense if considering multiple XWalkView instances
in one application. It'd better to share one devtools server for remote
debugging to avoid unncessary native resource allocation.

This patch is to make XWalkDevToolsServer be singleton and add reference
count to it for native object release.

BUG=https://crosswalk-project.org/jira/browse/XWALK-105