Fixed critical bug EXT-1655 ([BSI] Always flying after pressing "Stand" to stand up from an object)

 - moved restoring the flying of the agent before calling the mAutoPilotFinishedCallback, to allow finished callback to change it.
 - fixed LLVOAvatar::sitOnObject() to set mIsSitting = TRUE before stoping the autopilot, like it was in 1.23 version.
Reviewed by Mike Antipov at https://codereview.productengine.com/secondlife/r/185/

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branch : product-engine