Increase number of IP reachability probes and probing interval.

When neighbors on a wifi network (namely the default gateway or DNS
server) become unresponsive, we keep probing them for a total of 15
seconds (instead of the default 7.5s) before declaring them
unreachable. In practice, this allows TV devices to be more lenient
towards neighbors experiencing glitches, and avoids some cases of
spuriously disconnecting from wifi networks.

Test: cmd overlay dump --verbose
Test: cat /proc/sys/net/{ipv4,ipv6}/neigh/wlan0/{ucast_solicit,retrans_time_ms}
Change-Id: Ie8900895acc06b19635f1bfb5b6d19e605bb3a10
Bug: 163009579