Fix Sonos Line-in Action

Note: This is being changed to a pre-release as there may be a bug that causes the plugin to stop working for some users.

• actionZP_LineIn forwarded to actionDirect with action_id="ZP_LineIn", but no branch handled that id — it always fell through to the catch-all and logged Unknown or unsupported action: ZP_LineIn, doing nothing.
• Separately, the Line-In source dropdown (getZP_LineIn) read from a global list only ever written by getLineIn(), which nothing called — so the dropdown was always empty, both for this action and for the Group Announcement action's Line-In source field.
• Added a ZP_LineIn branch to actionDirect that resolves the selected source device's ZP_LocalUID and switches the zone via SetAVTransportURI (x-rincon-stream:), matching the pattern already used by the announcement code's working Line-In path.
• Rewrote getZP_LineIn to list live self.ZonePlayer devices instead of the dead Sonos_LineIn global.
• Removed the now fully-dead getLineIn() and Sonos_LineIn.