[Settings] Not allow APN delete when adding is not an option

When not allowing APN to add, user may not be able to recover easily
when delete it.

Therefore, avoid from APN to be deleted when adding is not allowed.

Bug: 243664439
Bug: 200875858
Test: local, robolectric
Change-Id: I5cf984000244b4ad901c6a4977a1368279323e0a