[Android] Fix the file picker crash issue.

This patch is to fix the special case with some ROMs.
If there is no SD card on device, getExternalStoragePublicDirectory()
returns null on these ROMs.
The ROMs changes the setting in "/system/etc/vold.fstab", external storage was
mounted on "/storage/sdcard1", but the default external storage was mounted from
"/storage/sdcard0".
If the default external storage was not existed, get the other storage.

BUG=XWALK-2902, XWALK-2972