audioflinger: exclude channel check for offload thread

When moving effect chain between two playback threads, there's a
channel count check for stereo only. Such check only applies for deep
buffer playback where we upmix or downmix all channel count to stereo.

However, this constraint is not applicable for offload threads.

CRs-Fixed: 745226
Change-Id: Iaa16ddef1c19bf8b1c600e8bdb1ee1568a3a45db