Allow configuration of DHCP lease time in tethering

Make lease time for DHCP offers configurable via
Settings.Secure.TETHER_LEASE_TIME which is either the lease time in
seconds or Settings.Secure.TETHER_LEASE_TIME_DEFAULT for the default
lease time defined by netd.

This change depends on Ic65818fdb472a49c72fb56b1ad3ac33e4136a9a4 and
I0b369889f7f790f43f9c89da0c4e133a342debd7.

Change-Id: I2a83242bcacccb55780716845d4f67728612cc74