Set proper initial values for many SensorProperties

SensorProperty in libksysguard was changed to reset the value of a
sensor on unsubscribe. However, this caused a number of "static" sensors
to break and only provide their value once. To fix this, SensorProperty
will now only reset the value if a valid initial value was provided.

This changes many SensorProperties to have valid initial values. This
has been applied primarily to sensors that are very volatile and which
provide a different value almost every update, like the various rate
sensors as well as the usage percentage sensors. For these sensors, it
is unlikely thata a historical value makes any sense.

Other sensors, like memory usage, are less volatile and will generally
not vary wildly, so providing a historical value for those seems better
than an empty value.

BUG: 446414