Fix NPE at Storage Settings configuration change

At configuration change, both StorageCategoryFragment and
ProfileSelectStorageFragment are re-created and there is
a different callback sequence than normal case.

This change save/restore mSelectedStorageEntry in instance
state bundle to fix the NPE.

Bug: 189108700
Test: manual
      Install work profile and rotate Storage Settings.
Change-Id: I39d7ef7ced22188ee66216d57518198d63671e0e