chore(#AlwaysOnMagnifier): Support feature flag flips observation

In AlwaysOnMagnificationFeatureFlag, support adding listener to DeviceConfig to
observe the feature flag flips. If the flag value is changed, notify
AccessibilityManagerService to enable/disable the feature.

Bug: 269414499
Test: manually set flag value with adb shell
      atest MagnificationControllerTest
      atest AccessibilityManagerServiceTest
Change-Id: Ie66972ae8a3ac0ebda2b888bf7265ec1360b6a40

services/accessibility/java/com/android/server/accessibility/AccessibilityManagerService.java

@@ -4567,6 +4567,17 @@ public class AccessibilityManagerService extends IAccessibilityManager.Stub
         return false;
     }
 
+    /**
+     * Called when always on magnification feature flag flips to check if the feature should be
+     * enabled for current user state.
+     */
+    public void updateAlwaysOnMagnification() {
+        synchronized (mLock) {
+            readAlwaysOnMagnificationLocked(getCurrentUserState());
+        }
+    }
+
+    @GuardedBy("mLock")
     boolean readAlwaysOnMagnificationLocked(AccessibilityUserState userState) {
         final boolean isSettingsAlwaysOnEnabled = Settings.Secure.getIntForUser(
                 mContext.getContentResolver(),

services/accessibility/java/com/android/server/accessibility/magnification/AlwaysOnMagnificationFeatureFlag.java

@@ -16,10 +16,13 @@
 
 package com.android.server.accessibility.magnification;
 
+import android.annotation.NonNull;
 import android.provider.DeviceConfig;
 
 import com.android.internal.annotations.VisibleForTesting;
 
+import java.util.concurrent.Executor;
+
 /**
  * Encapsulates the feature flags for always on magnification. {@see DeviceConfig}
  *
@@ -50,4 +53,39 @@ public class AlwaysOnMagnificationFeatureFlag {
                 Boolean.toString(isEnabled),
                 /* makeDefault= */ false);
     }
+
+    /**
+     * Adds a listener for when the feature flag changes.
+     *
+     * <p>{@see DeviceConfig#addOnPropertiesChangedListener(
+     * String, Executor, DeviceConfig.OnPropertiesChangedListener)}
+     */
+    @NonNull
+    public static DeviceConfig.OnPropertiesChangedListener addOnChangedListener(
+            @NonNull Executor executor, @NonNull Runnable listener) {
+        DeviceConfig.OnPropertiesChangedListener onChangedListener =
+                properties -> {
+                    if (properties.getKeyset().contains(
+                            FEATURE_NAME_ENABLE_ALWAYS_ON_MAGNIFICATION)) {
+                        listener.run();
+                    }
+                };
+        DeviceConfig.addOnPropertiesChangedListener(
+                NAMESPACE,
+                executor,
+                onChangedListener);
+
+        return onChangedListener;
+    }
+
+    /**
+     * Remove a listener for when the feature flag changes.
+     *
+     * <p>{@see DeviceConfig#addOnPropertiesChangedListener(String, Executor,
+     * DeviceConfig.OnPropertiesChangedListener)}
+     */
+    public static void removeOnChangedListener(
+            @NonNull DeviceConfig.OnPropertiesChangedListener onChangedListener) {
+        DeviceConfig.removeOnPropertiesChangedListener(onChangedListener);
+    }
 }

services/accessibility/java/com/android/server/accessibility/magnification/MagnificationController.java

@@ -48,6 +48,7 @@ import android.view.accessibility.MagnificationAnimationCallback;
 import com.android.internal.accessibility.util.AccessibilityStatsLogUtils;
 import com.android.internal.annotations.GuardedBy;
 import com.android.internal.annotations.VisibleForTesting;
+import com.android.internal.util.ConcurrentUtils;
 import com.android.server.LocalServices;
 import com.android.server.accessibility.AccessibilityManagerService;
 import com.android.server.wm.WindowManagerInternal;
@@ -149,6 +150,9 @@ public class MagnificationController implements WindowMagnificationManager.Callb
                 .getAccessibilityController().setUiChangesForAccessibilityCallbacks(this);
         mSupportWindowMagnification = context.getPackageManager().hasSystemFeature(
                 FEATURE_WINDOW_MAGNIFICATION);
+
+        AlwaysOnMagnificationFeatureFlag.addOnChangedListener(
+                ConcurrentUtils.DIRECT_EXECUTOR, mAms::updateAlwaysOnMagnification);
     }
 
     @VisibleForTesting