[Android] Fix crash on Android Crosswalk if no SD card on test device

Add code to check every directory to avoid crash.

BUG=XWALK-1931
(cherry picked from commit 11eb7f0aea763d22dd7b66843585a9688b20b543)
Merge pull request #2096 from shawngao5/crosswalk-7

[Android] Fix crash on Android Crosswalk if no SD card on test device