Correct new mailbox parent fixup code

* Change poorly named cursor variable (the code is confusing as it
  was)
* Fixup the parent of parentKey-less mailboxes so that all children
  are properly updated
* Fix faulty unit test (that otherwise would have caught this issue!)

Bug: 5794371
Change-Id: Ie249617775ba11d9f2e10528019893d4f9d9f81e
2 files changed