fd_utils: Fix broken usage of iterators.

There were two separate issues here :
- RestatInternal was using an iterator after a call to erase(). This
  will not work because it will be invalidated.
- The "standard" for loop idiom for iterating over a map while making
  structural changes to it is broken. Switch to a while loop and treat
  cases where elements are erased differently from cases where they
  aren't.

bug: 31092930
bug: 30963384
Change-Id: I261d59239558230dd8cdd1d1cb5b9e2448a4c23f