Resolve "WIP: Adding screenshare functionality"

Description

A screen share implementation preview:

Changes:

• Added screenCapture to Platform interface
• Implemented it in Android
• Changed/added local and remote video track publish/unpublish events to support multiple video tracks for one participant
• Added screen share button to Example

To be discussed:

• UX in example when using screen capture: notification, widgets and layout, change widget to full screen
• Approve adding of LocalVideoTrackUnpublishedEvent

Please note:

• It is buggy, testing was merely done on emulators.
• I’m new to this project, so please review and comment my changes
• Once approved I will clean up, diff and commit as expected
• I can add screen share to the web version, but need help for IOS

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting MRs.
• My MR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The flutter analyzer (flutter analyze) does not report any problems on my MR.
• The kotlin analyzer (ktlint) does not report any problems on my MR.
• The swift analyzer (swiftlint) does not report any problems on my MR.
• I read and followed the Flutter Style Guide.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy. And afterwards run flutter pub get, which will update the pubspec.lock with the correct new version.
• I updated CHANGELOG.md to add a description of the change.
  • And when a Twilio SDK is upgraded I also mentioned this in the CHANGELOG.md
• I am willing to follow-up on review comments in a timely manner.
• I am done with this MR and removed the Draft status, by clicking on the Mark as ready button in this MR

Breaking Change

Does your MR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

Related Issues

#70 (comment 568111987)

Closes #70