[Android] Add thread safety checking for XWalkView.

All XWalkView methods must be called on the UI thread.
Throw an exception when XWalkView methods are called from
other thread. So all unit test cases need to be refined for
this change.

BUG=https://crosswalk-project.org/jira/browse/XWALK-98
Merge pull request #1033 from junmin-zhu/thread

[Android] Add thread safety checking for XWalkView.