title: setCharacteristicNotification doesn't trigger onCharacteristicChanged
body: <p>I try to read peripheral's characteristic however <code>onCharacteristicChanged</code> is never called despite setting up <code>setCharacteristicNotification</code>.</p>
  
  <p>Method which gets my characteristic:</p>
  
  <pre><code>private void getCharacteristic(List&lt;BluetoothGattService&gt; gattServices) {
      if (gattServices == null) return;
  
      for (BluetoothGattService gattService : gattServices) {
          if (gattService.getUuid().toString().equals("000018f0-0000-1000-8000-00805f9b34fb")) {
              for (BluetoothGattCharacteristic gattCharacteristic : gattService.getCharacteristics()) {
                  if (gattCharacteristic.getUuid().toString().equals("00002af0-0000-1000-8000-00805f9b34fb")) {
                      mBluetoothLeService.setCharacteristicNotification(gattService.getCharacteristic(gattCharacteristic.getUuid()), true);
                  }
              }
          }
      }
  }
  </code></pre>
  
  <p>Setting up notifications:</p>
  
  <pre><code>public void setCharacteristicNotification(BluetoothGattCharacteristic characteristic,
                                                boolean enabled) {
          if (mBluetoothAdapter == null || mBluetoothGatt == null) {
              Log.w(TAG, "BluetoothAdapter not initialized");
              return;
          }
          mBluetoothGatt.setCharacteristicNotification(characteristic, enabled);
  }
  </code></pre>
  
  <p><code>onCharacteristicChanged</code> method which is never triggered:</p>
  
  <pre><code>@Override
  public void onCharacteristicChanged(BluetoothGatt gatt,
                                          BluetoothGattCharacteristic characteristic) {
          Log.d(TAG, "characteristic changed");
          broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);
  }
  </code></pre>
  
  <p>The same code works fine with different peripherals. </p>
  
  <p>I also downloaded nRF Connect app from Google Play store which displays characteristics and when i enabled notifications in nRF Connect, <code>onCharacteristicChanged</code> in my app started to get called (I could see "characteristic changed" in logcat).</p>