DnsResolver - improve aosp api level detection

This does a better job of handling non-cuttlefish,
ie. things like 'aosp_blueline' on aosp-master.

This will now detect rvc-dev as R, aosp cuttlefish as S,
and aosp blueline as R (should be S...).

Before this change aosp blueline would still be detected as Q.

Test: builds, atest, dnsresolver tests, TreeHugger
Bug: 159994981
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I81cb89d221dc19fe19e1acaebe75b953ea3cfaa6