Use at least one second for safe margin to drain.

When draining, there is not timestamp reported. The draining time is
based on the clock model, which estimates when the DSP may read all
data. It is good to have a bigger safe margin as the draining time
can be tens of seconds.

Bug: 433769515
Test: TH
Test: play offload stream, check xrun
Test: test_flush_from_frame -c2 -f2 -r48000
Flag: EXEMPT bugfix
Change-Id: Ib617050f79b9a9847601905139cecf2a5c96a665