Migrate local-album gallery widgets from pre-JB.

The path of external storage is changed to /storage/sdcard0 in JB and the bucket
IDs in MediaStore are changed accordinly. The local-album gallery widget is
saved by its bucket ID so this needs to be migrated to new value as well.

Bug: 6720251
Change-Id: I58a3044f2f05d428a9524f7a8053259950d5128d