Fix the live caption tooltip, which was sad and broken.

- Position it correctly using getLocationOnScreen since the dialog height is now variable.
- Update colors so it's readable in dark mode.
- Make it touchable so it can be dismissed without dismissing the entire volume dialog.

Fixes: 184624161
Test: commented out setting tooltip seen so I could experiment with it
Change-Id: I44ba0f9a5488edf88a63ff4842e9abc27e1ce408