Fix for #6541 that leaves preferences in place after conversion.

When you switch to Brackets 36, preferences will be converted automatically.
If you switch back to an earlier Brackets, your old preferences will still
be there. Any changes you make in that earlier Brackets will not be re-converted
to 36-style preferences.
use separate var names for convertedKeys & convertedKeysMap
Merge pull request #6550 from adobe/dangoor/6541-leave-prefs

Fix for #6541 that leaves preferences in place after conversion.