[Android] Fix crash caused by calling pauseTimer twice in row

The timer is actually global in current implementation.
Use a static boolean to record the status to avoid duplicated
calling pauseTimer().

BUG=https://crosswalk-project.org/jira/browse/XWALK-2078
Merge pull request #2178 from wang16/timer

[Android] Fix crash caused by calling pauseTimer twice in row