Disable screen lock setting instead of hiding it when not available

briar-android/src/main/java/org/briarproject/briar/android/settings/SettingsFragment.java

@@ -205,12 +205,7 @@ public class SettingsFragment extends PreferenceFragmentCompat
 		enableBluetooth.setOnPreferenceChangeListener(this);
 		torNetwork.setOnPreferenceChangeListener(this);
 		torBlocked.setOnPreferenceChangeListener(this);
-		if (getActivity() != null && hasScreenLock(getActivity())) {
-			screenLock.setVisible(FEATURE_FLAG_PIN_LOCK);
-			screenLock.setOnPreferenceChangeListener(this);
-		} else {
-			screenLock.setVisible(false);
-		}
+		screenLock.setOnPreferenceChangeListener(this);
 		if (SDK_INT >= 21) {
 			notifyLockscreen.setVisible(true);
 			notifyLockscreen.setOnPreferenceChangeListener(this);
@@ -231,6 +226,7 @@ public class SettingsFragment extends PreferenceFragmentCompat
 		} else {
 			theme.setVisible(FEATURE_FLAG_DARK_THEME);
 			notifySignIn.setVisible(FEATURE_FLAG_SIGN_IN_REMINDER);
+			screenLock.setVisible(FEATURE_FLAG_PIN_LOCK);
 
 			findPreference("pref_key_explode").setVisible(false);
 			findPreference("pref_key_test_data").setVisible(false);
@@ -428,7 +424,12 @@ public class SettingsFragment extends PreferenceFragmentCompat
 		enableBluetooth.setEnabled(enabled);
 		torNetwork.setEnabled(enabled);
 		torBlocked.setEnabled(enabled);
-		screenLock.setEnabled(enabled);
+		if (enabled && getActivity() != null && hasScreenLock(getActivity())) {
+			screenLock.setEnabled(true);
+		} else {
+			screenLock.setEnabled(false);
+			screenLock.setSummary(getString(R.string.lock_disabled));
+		}
 		notifyPrivateMessages.setEnabled(enabled);
 		notifyGroupMessages.setEnabled(enabled);
 		notifyForumPosts.setEnabled(enabled);

briar-android/src/main/res/values/strings.xml

@@ -452,5 +452,6 @@
 	<string name="lock_unlock">Unlock Briar</string>
 	<string name="lock_is_locked">Briar is locked</string>
 	<string name="lock_tap_to_unlock">Tap to unlock</string>
+	<string name="lock_disabled">Only available on Android 5 or newer and when a system screen lock is set</string>
 
 </resources>

briar-android/src/main/res/xml/settings.xml

@@ -61,7 +61,7 @@
 			android:persistent="false"
 			android:summary="@string/pref_lock_summary"
 			android:title="@string/pref_lock_title"
-			android:visibility="gone"/>
+			android:enabled="false"/>
 
 		<Preference
 			android:key="pref_key_change_password"