Only force DNS handling if there is DNS data

Currently Glean rewrites resolve.conf and disables DNS handling in
NetworkManager even if there is no DNS information provided, making it
impossible to e.g. build DNS servers into the image. This behavior
also does not play way with the --no-dhcp-fallback flag.

Change-Id: I59cf2ece4e665d569d1db08d1df19b0892c6ba9d

glean/install.py

@@ -152,12 +152,6 @@ def main():
             '/etc/udev/rules.d/99-glean.rules',
             mode='0644')
         if args.use_nm:
-            # glean handles resolv.conf, so this turns off nm touching
-            # it
-            install('nm-no-resolv-handling.conf',
-                    '/etc/NetworkManager/conf.d/nm-no-resolv-handling.conf',
-                    mode='0644')
-
             # NetworkManager has a "after" network-pre, and
             # glean@<interface> services have a "before".  However, if
             # udev has not yet triggered and started the glean