localvideo: refactor preview component device control

src/app/commoncomponents/LocalVideo.qml

src/app/currentcall.cpp

src/app/currentconversation.cpp

src/app/mainview/components/InCallLocalVideo.qml

src/app/mainview/components/OngoingCallPage.qml

src/app/settingsview/components/VideoSettingsPage.qml

src/app/videodevices.cpp

src/libclient/api/call.h

src/libclient/api/conversationmodel.h

src/libclient/conversationmodel.cpp

tests/qml/src/tst_OngoingCallPage.qml