Don't return error when uid entry don't exsit

When query per uid stats, do not return error if that uid does not have
any uid stats entry in AppUidStatsMap. It might be that uid doesn't
generate any traffic at all.

Bug: 124764595
Test: android.net.cts.TrafficStatsTest with wifi and data off.
      libnetdbpf_test
Change-Id: Ibe986e2d6ab362868c6b7b63b059111af0b5706f
Merged-In: I738dc376ae34cf6b1c62c14ffd2135d9b37f5925
Merged-In: Icd97c3cc3cbc05199d0a14b42a2acb094ccd77dc
(cherry picked from commit b9d8028f)