Tag with TAG_SYSTEM_DNS for dns packets.

Tag with TAG_SYSTEM_DNS for dns packets so that apps know that
the traffic is not sent by them but is sent by the system on
their behalf.

Test: atest resolv_integration_test
Bug: 132125333
Change-Id: I2e3502d75bd1a6a55f5e62a7fa23141e4cf2e375