systemActions: Create SensorProxy unconditionally

When plugging in a device with sensors that are unsupported by
iio-sensor-proxy, the proxy may quit so fast that the name disappears
from the bus before we get to construct the SensorProxy in response
to the name-appeared handler, resulting in the following warning:

JS ERROR: TypeError: this._sensorProxy is null
_sensorProxyAppeared/this._sensorProxy<@resource:///org/gnome/shell/misc/systemActions.js:217:17
_makeProxyWrapper/</<@resource:///org/gnome/gjs/modules/overrides/Gio.js:243:21

Address this by creating the proxy unconditionally instead of monitoring
the bus name, and using the g-name-owner property to determine whether
iio-sensor-proxy is active.

GNOME/gnome-shell#1357
4 jobs for !580 with fix-sensor-proxy-warning in 5 minutes and 14 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #327064
check_commit_log

00:00:50

 
  Build
passed #327065
build

00:02:16

 
  Test
passed #327066
test

00:02:07

passed #327067
test-pot

00:02:06