Improve compatibility with the W3C specification.

[wifisher]

Description

Improved compatibility with the W3C specification (https://w3c.github.io/speech-api/webspeechapi.html) and other improvements.

• Added voicechanged event.
• Error events from Android now include the error value and message.
• Removed SpeechSynthesisVoiceList.

Related Issue

Related to issue 18

Motivation and Context

How Has This Been Tested?

I have run the changes in a test app. The app code works the same for Android, iOS, Windows and Chrome.

Screenshots (if appropriate):

Types of changes

• 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)

Checklist:

• 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.
• I have added tests to cover my changes.
• All new and existing tests passed.