[Android] Fix native filesystem API initialize crash for cordova

When application doesn't declare "WRITE/READ_EXTERNAL_STORAGE" permission in
its Android.manifest, getExternalCacheDirectory() will still return null even
though sd card is mounted.For cordova usage, the permission is not added by default.

BUG=XWALK-1952
(cherry picked from commit dd63b0fe7c0793240042700d5ca5205b83f172e5)