Show/hide back button for 2nd page

By design, we want to show/hide back button for 2nd
layer page when it's in two pane mode.

Test: Manual test
Bug: 219000314
Change-Id: I14d8d7ee08280d5ce80c0b59e1e91d077f0b6d09

src/com/android/settings/development/featureflags/FeatureFlagPreference.java

@@ -17,6 +17,8 @@
 package com.android.settings.development.featureflags;
 
 import android.content.Context;
+import android.provider.Settings;
+import android.text.TextUtils;
 import android.util.FeatureFlagUtils;
 
 import androidx.preference.SwitchPreference;
@@ -49,5 +51,13 @@ public class FeatureFlagPreference extends SwitchPreference {
         } else {
             FeatureFlagUtils.setEnabled(getContext(), mKey, isChecked);
         }
+
+        // A temporary logic for settings_hide_secondary_page_back_button_in_two_pane
+        // Remove it before Android T release.
+        if (TextUtils.equals(mKey,
+                FeatureFlagUtils.SETTINGS_HIDE_SECONDARY_PAGE_BACK_BUTTON_IN_TWO_PANE)) {
+            Settings.Global.putString(getContext().getContentResolver(),
+                    mKey, String.valueOf(isChecked));
+        }
     }
 }