NetBpfLoad - log Tethering Apex version

(super useful for debugging)

Sample output:
  I NetBpfLoad: NetBpfLoad '/apex/com.android.tethering/bin/netbpfload' starting...
  I NetBpfLoad: NetBpfLoad api:10000/34 kver:6010019 platform:0 mainline:1
  D NetBpfLoad: Found Tethering Apex mounted from blockdev /dev/block/dm-55
  I NetBpfLoad: Tethering APEX version 340090000

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Iaf1bc22b02913ea21ff9368abd9933e6ef4017f1