Fixed normal bug EXT-6909 ('Mute/unmute' moderation options remain enaibled...

Fixed normal bug EXT-6909 ('Mute/unmute' moderation options remain enaibled after moderator left group call)

Reason: session's Speaker Manager was not updated when voice call ended. Speakers had incorrect voice related state.

Fix: update speakers' state when voice call is finished.
   Also VCP now uses LLIMMgr::endCall to perform "Leave Call" action.

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/276/

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branch : product-engine