Added net.dns1 prop for emulator, as Android emulator ignores it by default.

This commit sets up the system property which is actually used by the
Connectivity Service
(frameworks/base/services/java/com/android/server/ConnectivityService.java).
It fixes an (obsolete?) convention where the dns was affected directly by
the interface (i.e. "net.eth0.dns1=10.0.2.3"), which causes the Android
Emulator (goldfish) to ignore this value, and effectively have no DNS
resolving at all.

An immediate fix can be either add reference to net.eth%s.dns%s in the
ConnectivityService and possibly on the dhcp code as well which would be
bloated, or just stick to the apparant new convention.

I chose the latter as a one line fix which gets the job done.

Change-Id: Id4364129e9a82c1f48403068a837aca54de07944