Add "Touchpad Visualizer" to input developer options

Bug: 28231224
Test: atest TouchpadVisualizerPreferenceControllerTest
Test: Manual testing using
      $ adb shell settings get system touchpad_visualizer
Flag: com.android.hardware.input.touchpad_visualizer

Change-Id: I4f6b70b0da12253950b017363f3afcecc1a32570