audioflinger: fix standby delay on A2DP output

Make sure that standby delay is never less than the audio flinger
default on A2DP output.
Due to variable latency and amount of buffering in A2DP sinks,
an agressive standby delay could lead to truncated audio.

Bug: 25830539.
Change-Id: I38be37ad346f5f4bf8303d3db4e3e911bf637968
(cherry picked from commit 42537be6)