Let PowerState class run multiple callbacks

Previously the PowerState class could only handle a single callback
function. This commit changes that to allow other callback functions to
be added with

Let PowerState class run multiple callbacks

Previously the PowerState class could only handle a single callback
function. This commit changes that to allow other callback functions to
be added with a addCallback() method. The callback functions are then
executed when the power state changes.

This also changes the use of the PowerState object in the fan presence
code to a shared_ptr instead of a unique_ptr, and adds a function to
return the PowerState object to use in the app, creating it if
necessary. This allows multiple pieces of the code to add their own
callbacks to the same object.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I35fc8ab94c4806f0c7fd2f1552c58131b8f30f23

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