Don't prefer APNs with no type for initial attach.

When choosing an APN for initial attach, we currently treat an
APN with no type the same as an APN that explicitly sets a type
of "ia". Such an APN will take priority over the preferred APN,
and will even take priority over APNs with the type explicitly
set to "ia".

This breaks attach on networks where there are multiple
MVNOs with the same MCC/MNC and one or more of them has an APN
with no type.

This seems to be unintentional, because the comments say that
APNs with a type of "ia" are preferred over default APNs. Fix it
by only considering an APN to be an initial attach APN if it
explicitly specifies a type of "ia".

Bug: 10614153
Change-Id: I8ed81ab3b58ae2aab2c76b644b653bb2c08da163