Display "see all" option when bluetooth is off

- This CL before, Bluetooth settings will hide the "see all"
  option when bluetooth is off. For the UX improvement, this
  CL will show the "see all" button when bluetooth is off. And
  turn on the BT when user press "see all" button to let user to
  quick pair their disconnected devices.
- Update and add test case

Bug: 147150367
Test: make -j42 RunSettingsRoboTests
Change-Id: I7a1451a36a31253b6be2203341bea4e59fed7c2b

src/com/android/settings/connecteddevice/PreviouslyConnectedDeviceDashboardFragment.java

@@ -16,7 +16,11 @@
 package com.android.settings.connecteddevice;
 
 import android.app.settings.SettingsEnums;
+import android.bluetooth.BluetoothAdapter;
 import android.content.Context;
+import android.os.Bundle;
+
+import androidx.annotation.VisibleForTesting;
 
 import com.android.settings.R;
 import com.android.settings.dashboard.DashboardFragment;
@@ -32,6 +36,9 @@ public class PreviouslyConnectedDeviceDashboardFragment extends DashboardFragmen
     private static final String TAG = "PreConnectedDeviceFrag";
     static final String KEY_PREVIOUSLY_CONNECTED_DEVICES = "saved_device_list";
 
+    @VisibleForTesting
+    BluetoothAdapter mBluetoothAdapter;
+
     @Override
     public int getHelpResource() {
         return R.string.help_url_previously_connected_devices;
@@ -52,12 +59,32 @@ public class PreviouslyConnectedDeviceDashboardFragment extends DashboardFragmen
         return SettingsEnums.PREVIOUSLY_CONNECTED_DEVICES;
     }
 
+    @Override
+    public void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+
+        mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
+    }
+
     @Override
     public void onAttach(Context context) {
         super.onAttach(context);
         use(SavedDeviceGroupController.class).init(this);
     }
 
+    @Override
+    public void onStart() {
+        super.onStart();
+        enableBluetoothIfNecessary();
+    }
+
+    @VisibleForTesting
+    void enableBluetoothIfNecessary() {
+        if (mBluetoothAdapter != null && !mBluetoothAdapter.isEnabled()) {
+            mBluetoothAdapter.enable();
+        }
+    }
+
     /**
      * For Search.
      */