httplive: Set start time and segment start time in conjunction.

Also add comments describing how start time and segment start time are
used.

Based on AOSP CL https://android-review.googlesource.com/127653
by Joakim Johansson <joakim.c.johansson@sonymobile.com> but uses the
lowest segment start time instead of highest.

Bug: 18821145
Change-Id: I14cf1186d0daf517a24e8423c3a708b4c9ba06c4