[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 f843e446acb81cec8f1c2162630296c411f766ed)

Conflicts:
	runtime/android/core_internal/src/org/xwalk/core/internal/XWalkViewInternal.java