Never restricts Accessibility Activities based on ECM/admin.

Keeps accessibility activity preference as inheriting from
RestrictedPreference since it shares lots of behavior with the
accessibility service perference (same base class), but always
calls setEnabled(true) for activities.

Fix: 331990900
Flag: com.android.settings.accessibility.never_restrict_accessibility_activity
Test: atest RestrictedPreferenceHelperTest
Change-Id: I39971bc3f65aa630fa62a0e31132f177fd21b635

aconfig/accessibility/accessibility_flags.aconfig

@@ -61,6 +61,16 @@ flag {
   }
 }
 
+flag {
+  name: "never_restrict_accessibility_activity"
+  namespace: "accessibility"
+  description: "Stops possibly restricting AccessibilityActivityPreferences"
+  bug: "331990900"
+  metadata {
+    purpose: PURPOSE_BUGFIX
+  }
+}
+
 flag {
   name: "remove_qs_tooltip_in_suw"
   namespace: "accessibility"