[Android] Add back key support

1 The patch is to add the back key support for the runtime. The
expected behavior when putting on the back button is to go back
to the previous page, but now it results to exit the app.
2 Add a new goBack() interface into XWalkRuntimeView to handle this.
3 Hook the onKeyUp() method in XWalkRuntimeActivityBase and
XWalkRuntimeShellActivity to handle this.

BUG=https://crosswalk-project.org/jira/browse/XWALK-83