Tighten up neighbor timers if IpReachabilityMonitor is in use

Currently the kernel sends 3 probes, 1000ms apart when a neighbor is
in NUD_PROBE state (either naturally or artificially moved to that
state when, for example, confirming network configuration following
an AP association/roam).

With this change the device sends 5 probes at 750ms apart.  This
gives the network infrastructure a chance to respond to any of
5 probes instead of any of only 3, at a cost of increasing the total
probe time from 3s to 3.75s.

Future changes may read these values from settings (or from somewhere
else) so that experiments may be conducted or values adjusted based on
other information.

Test: as follows
    - built, flashed, booted
    - runtest frameworks-net passes
    - examinaton of /proc settings on wifi shows updated values
    - connected to a test network, disconnected the backhaul, ran
      "adb shell dumpsys wifi ipclient confirm" while watching
      "adb shell tcpdump -n -i wlan0 -l -vv arp", and observed
      5 ARP who-has ("solicits") ~750ms apart.
Bug: 62476366
Bug: 62845299
Change-Id: Ib805531b08624805bb63703a5b338b9ff48171d4