AudioFlinger: Make StandbyTime configurable.

Make the standby time for AudioFlinger configurable using a system
property.  Default AudioFlinger behavior is to go into standby
(allowing the audio outputs to underflow) after there has been nothing
to mix and AudioFlinger has just been pumping out silence for the
configured standby time (which defaulted to 3 seconds).

Now, by setting the "ro.audio.flinger_standbytime_ms" property in
their platform init.rc, platforms can override this default and
control the standby time.  If the property is missing or malformed,
the old default value of 3 seconds will be used instead.

Change-Id: Ic9fa8b5f5bccee493bc72c65e408d3fd8ddd1059
Signed-off-by: John Grossman <johngro@google.com>