Fix binder log tests

This is a follow-up of aosp/1579804, which removed "(null)" from
output logs. This change does the same tidy-up with
android::base::StringReplace() which can replace every occurrence. This
is more future-proof in case there's more than one nullable arguments.

Bug: n/a
Test: atest DnsResolverBinderTest
Change-Id: Iee2d0ef4717c5ff69ab469997ea9c30d0437544a