[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
Merge pull request #2112 from shawngao5/XWALK-1952-master

[Android] Fix native filesystem API initialize crash for cordova