[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 edb645aea9859a65a48e4662a6e0c8a0ffd7cc2d)