audioflinger: fix ro.audio.silent in offload again

Commit 1abbdb44 was not working 100% of the times because
it is possible that the offload thread loop never sleeps
after being created in which case the property is never read.

The loop now reads the property once when starting.

Bug: 10899309.
Change-Id: I2e2ca332f2d7086e59d65f6010378c4d9618ba9e