Sync parent directory in storeKeyAtomically()

When an FBE or metadata encryption key is created, it's important that
it be persisted to disk right away; otherwise the device may fail to
boot after an unclean shutdown.  storeKey() has the needed fsync()s.
However, storeKeyAtomically() doesn't, as it doesn't fsync() the parent
directory of key_path after it renames tmp_path to it.

Two callers do fsync() the parent directory themselves, but others
don't.  E.g., the metadata encryption key doesn't get properly synced.

Therefore, add the needed fsync() to storeKeyAtomically() so that it
gets done for everyone.

Also remove the now-unneeded fsync()s from the two callers that did it
themselves.

Change-Id: I342ebd94f0a3d2bf3a7a443c35b6bda0f12e1ab2