Fix virtual tooltip changeMessge handler not checking reference

[stefanbalmus]

Description

This fixes a bug in Theme.ts which leads to md-tooltip components showing the wrong tooltip. The issue can be observed in several ways but one example is:

• have an md-audio-player component
• immediately upon displaying, hover mouse over Play button
• observe the correct "Play" tooltip
• wait for the audio src to load (displays audio file duration)
• once audio src is loaded the tooltip for the Play button changes to "Volume level 75%"

Related Issue

Motivation and Context

How Has This Been Tested?

To reproduce the issue easier, I've added a delay (during development testing) in AudioPlayer.ts inside the firstUpdated so that the audio source is updated with a delay. This is to allow enough time for the original tooltip to display, before the audio is loaded. Then followed the steps described above.

Screenshots:

Before (If applicable):

After:

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation update
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.