sink: avoid deadock during frame size change

Lock was:
    frame #3: 0x00000074f22d0b74 libc++_shared.so`std::__ndk1::mutex::lock()
    frame #5: 0x00000074f2f91b10 libring.so`std::__ndk1::lock_guard<std::__ndk1::mutex>::lock_guard()
    frame #6: 0x00000074f3156d3c libring.so`jami::video::SinkClient::registerTarget() at sinkclient.h:80:37
    frame #7: 0x00000074f3154f74 libring.so`DRing::registerSinkTarget(sinkId="2920319278288091") at videomanager.cpp:527:15
    frame #8: 0x00000074f2f1dccc libring.so`Java_net_jami_daemon_JamiServiceJNI_unregisterVideoCallback()
    frame #45: 0x00000074f2f3491c libring.so`SwigDirector_VideoCallback::decodingStopped()
    frame #48: 0x00000074f319a480 libring.so`void jami::emitSignal<DRing::VideoSignal::DecodingStopped>() at ring_signal.h:66:13
    frame #49: 0x00000074f31993ec libring.so`jami::video::SinkClient::setFrameSize(this=0x00000074c298caa0, width=0, height=0) at sinkclient.cpp:482:9
    frame #50: 0x00000074f3199f74 libring.so`jami::video::SinkClient::update() at sinkclient.cpp:427:13
    frame #51: 0x00000074f316c69c libring.so`jami::Observable<std::__ndk1::shared_ptr<DRing::MediaFrame> >::notify() at observer.h:138:23
    frame #52: 0x00000074f31a7094 libring.so`jami::video::VideoGenerator::publishFrame() at video_base.cpp:56:5


Change-Id: I1824bad767543a3e789e13af1489613a044a7473