Use FooterPreference in xml explicitly

We don't rely on FooterPreferenceMixinCompat
to create footer preference in dashboard fragment.

Instead, declare a FooterPreference explicitly in
xml of screen.

Test: visual, robotest
Bug: 124129485
Change-Id: I4c5b60c3926583eb0d9f0d4cd6996bf169d6408c

tests/robotests/src/com/android/settings/connecteddevice/ConnectedDeviceDashboardFragmentTest.java

@@ -51,6 +51,7 @@ import java.util.List;
         ShadowConnectivityManager.class, ShadowBluetoothAdapter.class})
 public class ConnectedDeviceDashboardFragmentTest {
     private static final String KEY_NEARBY_DEVICES = "bt_nearby_slice";
+    private static final String KEY_DISCOVERABLE_FOOTER = "discoverable_footer";
 
     @Mock
     private PackageManager mPackageManager;
@@ -81,7 +82,7 @@ public class ConnectedDeviceDashboardFragmentTest {
                 .getNonIndexableKeys(mContext);
 
         assertThat(niks).containsExactly(KEY_CONNECTED_DEVICES, KEY_AVAILABLE_DEVICES,
-                KEY_NEARBY_DEVICES);
+                KEY_NEARBY_DEVICES, KEY_DISCOVERABLE_FOOTER);
     }
 
     @Test