[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

(cherry picked from commit 0dd21390c49a7260b5b11e7e6c83539e97e28898)
Merge pull request #2599 from fujunwei/merge-exit-app

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