[Power] Send change events only if there's someone listening

[Power] Send change events only if there's someone listening

Add a new message SetListenToScreenStateChange, from JS to native, that
will be used to tell native that we have a listener. This patch also
ensures that we have a proper value to be used as "old state", by
getting the value at time of listener registration.

One consequence is that after this patch, tizen.power code is not called
at all unless the page uses tizen.power features.

This patch should reduce a lot the errors related to invalid sync
messages, they were caused mostly by those setup messages when running
tests that loaded pages repeatedly.