[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 072c9f2ef76368ba299fd553a4daa6e9fa9160c8)