Really always allow networking on loopback.

https://android-review.googlesource.com/#/c/294359/ attempted to
allow networking on loopback, but actually does not do anything
because no packet has both -i lo and -o lo: loopback packets have
-i lo in INPUT and -o lo in OUTPUT.

(cherry picked from commit d7e2e8a1)

Test: bullhead builds, boots
Test: netd_{unit,integration}_test pass
Test: loopback traffic is matched by new "-i lo" and "-o lo" rules
Test: originated and received traffic is not matched by new rules
Bug: 34444781
Change-Id: Ib7f1d04ea4ae85506c2b0a87bd5aa1378057f07f