adaptivedemux: use GstSystemClock to all real-time calculations

adaptivedemux: use GstSystemClock to all real-time calculations

A realtime clock is used in many places, such as deciding which
fragment to select at start up and deciding how long to sleep
before a fragment becomes available. For example dashdemux needs
sample the client's estimate of UTC when selecting where to start
in a live DASH stream.

The problem with dashdemux calculating the client's idea of UTC is
that it makes it difficult to create unit tests, because the passage
of time is a factor in the test.

This commit changes dashdemux and adaptivedemux to use the
GstSystemClock, so that a unit test can replace the system clock when
it needs to be able to control the clock.

This commit makes no change to the behaviour under normal usage, as
GstSystemClock is based upon the system time.

https://bugzilla.gnome.org/show_bug.cgi?id=762147