power: Only install callback once

A mode change to normal causes iio_proxy_claim_light() to be called.
Prior to this patch, this would always connect iio_proxy_changed_cb(),
even if the previous mode was dim and the callback was already
connected.  This resulted in the callback triggering multiple times on
each change of the ambient light sensor.  This patch adds a flag to
GsdPowerManager to track whether or not the callback is already
connected.
6 jobs for iio-proxy-cb in 3 minutes and 21 seconds
Status Job ID Name Coverage
  Build
passed #808452
build

00:00:51

 
  Test
passed #808453
test

00:02:29

 
  Manual
manual #808454
privileged allowed to fail manual
asan
manual #808457
allowed to fail manual
msan
manual #808455
allowed to fail manual
tsan
manual #808456
allowed to fail manual
ubsan