Add touchpad selection dropdown menu

Add a drop-down menu next to the touchpad name that,
when clicked, displays a list of all connected touchpads.
This menu allows the user to easily switch between any
available touchpad by selecting from the list.

Bug: 368239574
Test: Manual testing by verifying that the touchpad
      switch logic works as intended and that the dropdown
      menu is updated whenever a touchpad is added/removed.
Flag: com.android.hardware.input.touchpad_visualizer

Change-Id: Ia70072d681e78a9cbf998e126095b6da171939b7