Fixed the incorrect APN dedupling

Fixed that APN settings can't be merged when one APN
entry has MTU set, but the other doesn't. Also default
MTU unset value to 0 for backwards compatability.

Test: atest DataProfileManagerTest & Manual testing
Fix: 226563054
Change-Id: I02190be62855f5f3b86c41677bbf7ff6a362e652