Seek bar can be tapped on mobile while hidden

[joeyparrish]

Have you read the FAQ and checked for duplicate open issues?
Yes

What version of Shaka Player are you using?
v3.0.4

Can you reproduce the issue with our latest release version?
Yes

Can you reproduce the issue with the latest code from master?
Yes

Are you using the demo app or your own custom app?
Demo

What browser and OS are you using?
ChromeOS 85 stable with mobile emulator
Chrome 85 stable on Android

What are the manifest and license server URIs?
Any

What did you do?
While the controls are hidden, tap on the area of the seek bar

What did you expect to happen?
The controls should be shown before an individual control will respond to touch events

What actually happened?
The seek bar activated, allowing me to seek to another part of the video without being able to see the seek bar. This could easily lead to accidental activation.

Discovered while looking at #2886