DnsResolver apex - add a NetBpfLoad-S.flag for bpf_existence_test

(really should have been part of the previous commit
  'DnsProxyListener: init (but ignore failure) on S/Sv2'
since it serves as a flag that that patch is present)

With this we have:
  adb shell ls -al /apex/com.android.resolv/NetBpfLoad-S.flag
  -rw-r--r-- 1 system system 0 1970-01-01 00:00 /apex/com.android.resolv/NetBpfLoad-S.flag

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I2de1ca6adb5d139947b59d8ab8fefbbe14ad0a72