Tooltip on mobile should go away when I tap outside of canvas

## Expected Behavior
When I tap outside of a canvas tooltip should disappear, akin to what happens when I tap inside a canvas.
Or similarly to what happens on the desktop when I move mouse away from a circle.

## Current Behavior
Tooltip stays visible when I tap outside of a canvas

## Steps to Reproduce
Navigate to any tooltip example on mobile phone and tap on any data point.
You will see a tooltip.
Then tap outside of canvas and you will see that tooltip stays on a screen.

## Context
It's expected that tooltips and hints are transient elements and they go away as soon as user leaves the context when they appeared.

## Environment

* Chart.js version: 2.9.3
* Browser name and version: Safari on iOS 3.6.1 and Firefox Mobile 28.0
* Link to your project: https://www.chartjs.org/samples/latest/tooltips/custom-line.html