Only replace meta-data from dialog if it has changed

Previously we had a defaultSortKeyChanged value that lied - it was
possible for the value to be changed A -> B -> A by the user mid-
edit. However, the meta dialog assumed that defaultSortKeyChanged
wasn't lying, so blindly changed the meta item to the new value,
causing an unnecessary meta change if the user had done a no-op.

Now the value is renamed to defaultSortKeyTouched, and we actually
detect for content changes, and only change the meta item if a
change is actually needed (be that a removal, a replacement, or an
insertion).

Change-Id: I13022090bd7561a460a1151013e2b7d2a029f4dd
Merge "Only replace meta-data from dialog if it has changed"