sharing: Fix master switch

Since the switches for media, personal file, and screen sharing are in the header bar of the
respective dialogs, they are not visible when we click on the master switch. This caused the
check in OFF_IF_VISIBLE to always fail.

Fix it by checking if the corresponding button in the main list box is visible, instead.

Closes: #240
11 jobs for fix-sharing-master-switch in 3 minutes (queued for 5 seconds)
Status Job ID Name Coverage
  Build
passed #354556
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  Test
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