adapter: Implement PowerState property

adapter: Implement PowerState property

This property should allow any program to show whether an adapter is in
the process of being turned on.

As turning on an adapter isn't instantaneous, it's important that the UI
reflects the transitional state of the adapter's power, and doesn't
assume the device is already turned on but not yet working, or still off
despite having requested for it to be turned on, in both cases making
the UI feel unresponsive.

This can also not be implemented in front-ends directly as, then,
the status of an adapter wouldn't be reflected correctly in the Settings
window if it's turned on in the system menu. Implementing it in the
front-ends would also preclude from having feedback about the state of
the adapter when bluetoothd is the one powering up the adapter after the
rfkill was unblocked.

See https://gitlab.gnome.org/GNOME/gnome-bluetooth/-/issues/121
and the original https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5773

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>