shill: add function to check time till DHCP lease renewal

Add public function IPConfig::TimeToLeaseExpiry to check the time
left before the current DHCP lease is due to be renewed.

BUG=chromium:426657
TEST='P2_TEST_FILTER="shill::*" FEATURES="test" USE="clang asan"
emerge-samus shill' succeeds.

Change-Id: I80a912e86c677a98b48a22494e7bbc36a936cf3b
Reviewed-on: https://chromium-review.googlesource.com/225278
Reviewed-by: Samuel Tan <samueltan@chromium.org>
Commit-Queue: Samuel Tan <samueltan@chromium.org>
Tested-by: Samuel Tan <samueltan@chromium.org>
8 files changed