Unify log tag of DNS resolver

Set LOG_TAG to "resolv" on DNS resolver. Then, the log can be filtered
by "adb logcat [filterspecs]", such as "adb logcat *:S resolv:*", or
'grep'.

Test: resolv_integration_test pass
Change-Id: Ic4f97685d487257560ec3dfe62bab0d187bf9d93