Disable display of media controls on lock screen (showMediaControls) on KDE Plasma 5.12.7

The KDE lock screen has media player controls that look like this:

I would like to turn this feature off, as described in D9685 (Add config for the lockscreen to toggle media controls) and KDE Bug 384264.

I am running KUbuntu 18.04 with KDE Plasma 5.12.7, and my lock screen config file looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<kcfg xmlns="http://www.kde.org/standards/kcfg/1.0"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.kde.org/standards/kcfg/1.0
      http://www.kde.org/standards/kcfg/1.0/kcfg.xsd" >
  <kcfgfile name=""/>

  <group name="General">
    <entry name="showMediaControls" type="Bool">
      <label>If true, shows any currently playing media along with controls to pause it.</label>
      <default>true</default>
    </entry>
  </group>

</kcfg>

/usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/lockscreen/config.xml

Unlike older versions, the media controls are configurable in Plasma 5.12.7:

$ grep -B 3 -A 2 config.showMediaControls /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/lockscreen/LockScreenUi.qml
Loader {
    Layout.fillWidth: true
    Layout.preferredHeight: item ? item.implicitHeight : 0
    active: config.showMediaControls
    source: "MediaControls.qml"
}

Accordingly, I have tried to change the showMediaControls setting, but the setting is not available in the screenlocker menu:

$ kcmshell5 screenlocker

Changing the theme from KUbuntu to Breeze or Breeze Dark had no effect.

I also tried changing the configuration from the command line. Since the name attribute of the kcfgfile element is empty, I would expect the config file to be lockscreenrc, so I used this command:

kwriteconfig5 --file lockscreenrc --group 'General' --key 'showMediaControls' --type 'bool' 'false'

However, this had no effect. Even after writing this setting to all these files:

• breezerc
• kcmshell5rc
• kdeglobals
• lockscreenrc
• plasmarc

the player controls are still visible on the lock screen. How can I actually remove the controls from the lock screen?