Fix values to disable brightness reduction (MR 12)

These lines were implemented in the xfce4-phone config to simplify the
display power behavior. They set the number of seconds until Xfce4 power
manager reduces the display brightness.

The idea is to disable the brightness reduction by Xfce4 power manager
because this can confuse the tool "brightnessctl", which offers a service
to save and restore the display brightness across boots.

A value of 0 did not achieve the desired effect. The display brightness got
reduced nonetheless.

It turned out that the minimum time is 10 secs. Because of that a value of
9 disables the brightness reduction feature.

config/xfce4/xfconf/xfce-perchannel-xml/xfce4-power-manager.xml

@@ -4,8 +4,8 @@
   <property name="xfce4-power-manager" type="empty">
     <property name="show-tray-icon" type="bool" value="false"/>
     <property name="show-panel-label" type="int" value="1"/>
-    <property name="brightness-on-battery" type="uint" value="0"/>
-    <property name="brightness-on-ac" type="uint" value="0"/>
+    <property name="brightness-on-battery" type="uint" value="9"/>
+    <property name="brightness-on-ac" type="uint" value="9"/>
     <property name="blank-on-battery" type="int" value="0"/>
     <property name="blank-on-ac" type="int" value="0"/>
     <property name="dpms-on-battery-sleep" type="uint" value="0"/>