[Android] App crashed when invoking navigator.app.exitApp

The XWalkContent was destroyed when mNativeContent set to 0 in destroy() function.
Don't invoke the webContents.stop after that.
Add similar check to other related APIs.

BUG=XWALK-2815
Merge pull request #2558 from fujunwei/exit-app-crash

[Android] App crashed when invoking navigator.app.exitApp