Remove the video tile mapping entry when video is removed instead of when video is unbound

[tianyux-amz]

Issue #, if available:

aws/amazon-chime-sdk-android#186

Description of changes:

Changed SDK behavior to remove the internal video tile mapping entry when video is removed, instead of when video is unbound. This provides better API symmetry so that the video metadata will be added in videoTileDidAdd(tileState) callback and removed in videoTileDidRemove(tileState) callback.

Testing done:

Tested by modifying the demo app so that it does not bind remote video in videoTileDidAdd(tileState) and it does not unbind remote video in videoTileDidRemove(tileState). Test steps:

1. UserA joined Web client and enabled video.
2. UserB joined mobile client. Verified in the log that videoTileDidAdd was called for UserA's video, but the view was not bound (so video was not showing on the screen).
3. UserA disabled video. Verified in the log that videoTileDidRemove was called for UserA's video.
4. UserA enabled video again. Verified in the log that videoTileDidAdd was called again for UserA's video.

Manual test cases (add more as needed):

• Join meeting without CallKit
• Join meeting with CallKit as Incoming
• Join meeting with CallKit as Outgoing
• Leave meeting
• Rejoin meeting
• See metrics
• See attendee presence
• Send audio
• Receive audio
• Mute self
• Unmute self
• See local mute indicator when muted
• See remote mute indicator when other is muted
• Enable and disable local video
• Enable and disable remote video from remote side
• Send local video
• Pause and resume remote video
• Switch local camera
• Receive remote screen share
• Unit tests pass

Screenshots, if available:

N/A

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.