Shorten the timeout of DHCP INIT-REBOOT state.

Currently the timeout in the INIT-REBOOT state is 18s which allows
4 retransmits. In the scenario where DHCP server fails to response
due to certain reason, DhcpClient should go back to INIT state ASAP
and do the regular DHCPDISCOVER. Shorten the timeout to 5s to speed
up the fallback process.

Bug: 157960451
Bug: 157603386
Test: atest NetworkStackIntegrationTests
Test: manual test to verify that client goes back to INIT state
      and broadcasts DHCPDISCOVER after 2 retransmissions.
Change-Id: I9d214e193a4f80a9512a3a6f385ca3554f050386