lock screen: use device admin instead of accessibility service

app/src/main/res/values/strings.xml

@@ -1,18 +1,13 @@
 <resources>
-    <string name="accessibility_service_description">Enable lock screen action</string>
     <!--
-    -
-    - Home
-    -
-    -->
+      -
+      - Home
+      -
+      -->
     <string name="alert_cant_open_title">Can\'t open app</string>
     <string name="alert_cant_open_message">Want to change its settings?</string>
-    <string name="alert_lock_screen_failed">Failed to lock screen</string>
-
 
     <string name="toast_cant_open_message">Open settings to choose an action for this gesture</string>
-    <string name="toast_accessibility_service_not_enabled">μLauncher\'s accessibility service is not enabled. Please enable it in settings</string>
-    <string name="toast_lock_screen_not_supported">Sorry, lock screen is available on Android 9+</string>
 
     <!--
       -
@@ -207,11 +202,13 @@
     <string name="tutorial_finish_button">Start</string>
 
 
-
     <string name="settings">Settings</string>
     <string name="ic_menu_alt">More options</string>
     <string name="alert_cant_expand_status_bar_panel">Error: Can\'t expand status bar.\nThis action is using functionality that is not part of the published Android API. Unfortunately, it does not seem to work on your device.</string>
     <string name="snackbar_app_hidden">App hidden. You can make it visible again in settings.</string>
     <string name="undo">Undo</string>
     <string name="list_other_expand_settings_panel">Quick Settings</string>
+    <string name="toast_device_admin_not_enabled">µLauncher needs to be a device admin in order to lock the screen.</string>
+    <string name="device_admin_explanation">This is required for the lock screen action.</string>
+    <string name="device_admin_description">Enable the lock screen action</string>
 </resources>

app/src/main/res/xml/accessibility_service_config.xml

@@ -1,7 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<accessibility-service xmlns:android="http://schemas.android.com/apk/res/android"
-    android:accessibilityEventTypes="typeAllMask"
-    android:accessibilityFeedbackType="feedbackAllMask"
-    android:accessibilityFlags="flagDefault"
-    android:description="@string/accessibility_service_description"
-    />

app/src/main/res/xml/device_admin_config.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<device-admin xmlns:android="http://schemas.android.com/apk/res/android">
+    <uses-policies>
+        <force-lock />
+    </uses-policies>
+</device-admin>