fallbackswitch: Fix pad health calculation and notifies

Change the pad health calculation to consider a pad 'healthy' if it has received data within the last 'timeout' window. Previously, inactive pads were constantly flip-flopping between healthy and not healthy depending on whether they were slightly ahead of or behind the active pad running_time.

When the health status of a pad changes, make sure to always notify the property, so that applications that are manually controlling the active pad can make their switching decisions.

This is a fairly intrusive patch, so it would be good to get some review and testing from people using fallbackswitch in their scenarios. @heftig @vivia

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