Update preference screen title.

- Add missing title to preference screen xml so that they will be used to
set the activity title when the fragment is launched.
- Also updated some incorrect preference screen titles.
- Overrides getTitle() in preference fragments that do not use the
preference screen xml.

Bug: 64564191
Test: blaze-bin/screenshots/android/i18nscreenshots/i18nscreenshots
Change-Id: Id72d5ddf18f0962bc484de8bbd847a2e55d6371e

tests/robotests/src/com/android/settings/development/featureflags/FeatureFlagsDashboardTest.java

@@ -51,8 +51,8 @@ public class FeatureFlagsDashboardTest {
     }
 
     @Test
-    public void shouldUsePlaceholderPreferenceLayout() {
+    public void shouldUseFeatureFlagPreferenceLayout() {
         assertThat(mDashboard.getPreferenceScreenResId())
-                .isEqualTo(R.xml.placeholder_prefs);
+                .isEqualTo(R.xml.feature_flags_settings);
     }
 }