Fix demo button camera toggle to retain existing screen selection

[devalevenkatesh]

Issue #:
Camera option selected from Device selection page is retained in
the meeting whenever video turned off and on in JS SDK meetingV2 demo.

When we toggle the button camera we choose the video input device value
from the select box which is not in the in-meeting screen rather it is
in the preview device selection screen.

The button-camera button is on the in-meeting screen and hence since it
is selecting "video-input" (preview video input HTML select box) value
it is picking and retaining what you select on the preview screen and
not retaining in-meeting screen selection.

Description of changes:
Fix is to use stored selected video input over using video input from
preview device selection page.

Testing:

1. Join a meeting.
2. On device selection screen select a device.
3. Click continue and go to in-meeting screen, enable local video, check the selected device is from the preview screen.
4. No select a different video input device (external cam or virtual cam).
5. Turn off the local video and turn ON the local video again you should see the video input from last selected video input device. (Earlier it was picking whatever we selected on the preview screen).

Can these tested using a demo application? Please provide reproducible step-by-step instructions.
meetingV2

Checklist:

1. Have you successfully run npm run build:release locally?
   Demo change.

2. Do you add, modify, or delete public API definitions? If yes, has that been reviewed and approved?
   NA

3. Do you change the wire protocol, e.g. the request method? If yes, has that been reviewed and approved?
   NA

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.