Unexpected crash when run/debug xwalkdriver unit tests

Add code to check the conditions of empty App icon. Since the runtime::app_icon() will
return an empty icon for Android non-host-application.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2914
Merge pull request #2636 from iKevinHan/devtools_cause_crash_android

[Android] Fix devtools favicon will cause crash.