Fix two parsing bugs in new DHCP client.

1. We don't parse PAD options properly, leading in failure to
   parse packets sent by DHCP servers that put the end of options
   marker after pad options and at an odd offset.
2. We get the DhcpResults vendorInfo from the wrong option type
   (60 instead of 43).

Fix these and add unit tests for the offer packets sent by a few
different DHCP servers.

Bug: 21955617
Bug: 22281295
Change-Id: I5d13f1a6a3ff0b53112f18f3db8792fa32ad2da3