QSoundEffect: Set playing to true before starting the audio-device.

When the audio-device starts playing it should be able to read from the
source immediately. The OpenSL plugin would fail because the source was
blocking the audio-device from reading, as the state still was in its
initial state (stopped).

Change-Id: Ia26fa2eb442e239f5209947e6c9cd464b7d976a4
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>