[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.

This patch is to make XWalkDevToolsServer be singleton. Right now, it is
only created the first time when 'getInstance' is called with the given
socket name.

BUG=https://crosswalk-project.org/jira/browse/XWALK-105