Zone expander cable power transfer

[Lohrer]

Currently enough power is transferred via the expansion cable to power the Preamp's micro. For example: main unit is plugged into mains power, zone expander is unplugged from mains power but has the expansion cable plugged in. The Zone Expander's micro will be on and the front-panel LEDs will be (weakly) on in this situation. There don't seem to be any major issues with this electrically but the LEDs being on is potentially confusing.

As a mitigation for now the Main Unit should be powered on last to avoid weird AmpliPi states.