Fix an issue that the stream does not start properly on the remote side when the video is turned off

[miyamoto83]

Fix an issue that the stream does not start properly on the remote side when the video is turned off.

This pull request introduces support for dummy video streams to address compatibility issues in certain browsers where audio streams require accompanying video streams to play correctly. Key changes include adding logic for creating and disposing of dummy canvas streams, modifying conditions for video track handling, and ensuring proper cleanup of dummy streams.

Dummy Video Stream Support:

• Added videoIsDummy property to LocalTracks class: This property tracks whether the current video stream is a dummy stream. ([static/js/index.jsR68](https://github.com/NII-cloud-operation/ep_webrtc/pull/1/files#diff-545ec6b75710fc34567f877877c541f0924c8ddde8f96af69489209ef551eb94R68))
• Introduced dummy canvas stream creation and disposal methods: Implemented createDummyCanvasStream and disposeDummyCanvasStream methods to generate and clean up a dummy video stream using a canvas element. ([[1]](https://github.com/NII-cloud-operation/ep_webrtc/pull/1/files#diff-545ec6b75710fc34567f877877c541f0924c8ddde8f96af69489209ef551eb94R531-R575), [[2]](https://github.com/NII-cloud-operation/ep_webrtc/pull/1/files#diff-545ec6b75710fc34567f877877c541f0924c8ddde8f96af69489209ef551eb94R644))
• Integrated dummy video stream handling in track management: Updated logic to add a dummy video stream when no live video tracks are present and set videoIsDummy accordingly. ([[1]](https://github.com/NII-cloud-operation/ep_webrtc/pull/1/files#diff-545ec6b75710fc34567f877877c541f0924c8ddde8f96af69489209ef551eb94L451-R456), [[2]](https://github.com/NII-cloud-operation/ep_webrtc/pull/1/files#diff-545ec6b75710fc34567f877877c541f0924c8ddde8f96af69489209ef551eb94R497-R499))

Compatibility Improvements:

• Modified playVideo method: Updated conditions to specifically check for enabled video tracks, preventing indefinite blocking when no video tracks are active. ([static/js/index.jsL629-R679](https://github.com/NII-cloud-operation/ep_webrtc/pull/1/files#diff-545ec6b75710fc34567f877877c541f0924c8ddde8f96af69489209ef551eb94L629-R679))

Activation Logic Adjustment:

• Added _activated check in RTC message handling: Ensured that RTC messages are processed only when the RTC instance is activated. ([static/js/index.jsL331-R336](https://github.com/NII-cloud-operation/ep_webrtc/pull/1/files#diff-545ec6b75710fc34567f877877c541f0924c8ddde8f96af69489209ef551eb94L331-R336))