Run dnsmasq as dns_tether rather than nobody

AID_DNS_TETHER from android_filesystem_config.h is found and made
"friendly" by fs_config_generator.py (strip off AID_ and lowercase).

Test: as follows
    - built
    - flashed
    - booted
Bug: 29881876
Bug: 70673901
Change-Id: I3335a40dcd8c276421a9d663b7e66aef3a13b46b