qtdemux: Don't push GAP event if first buffer is within 1s

qtdemux: Don't push GAP event if first buffer is within 1s

If we saw empty segments, we previously unconditionally pushed a
GAP event downstream regardless of the duration of that empty
segment.

In order to avoid issues with initial negotiation of downstream elements
(which would negotiate to something before receiving any data due to
that initial GAP event), check if there's at least a second of difference
(like we do for other GAP-related checks in qtdemux) before
deciding to push a GAP event downstream.

Change-Id: I30318848c6f16cd04cf5bc776b77acd8ec0f3d0e