audioflinger: keep wakelock during offload drain

Even if ro.audio.offload_wakelock property is false we must
keep wakelock during offload drain seuqnce due to some driver
implementations risking to let the system enter suspend.

This restores functionality in commit 078538cf.

Bug: 30151948
Change-Id: Ib3a3e5c7e173ad7b1776817b81427932bfd16bf0