Set Default APN radio button's size to 48dp

Which is the minimum tap size.

Also set ApnPreference not selectable, since itspPrimary target and
radio button could be selectable, but entire preference itself is not
selectable.

Fix: 330812055
Fix: 330290274
Test: manual - with APNs
Change-Id: I651642e8a275932be6b8bcb365198e5dc1ad92f8