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: Ie03959e317d11e906e2f72833255a29c96920890
Merged-In: Ie03959e317d11e906e2f72833255a29c96920890