Fix keyframe attribute of interpolated shapes

[zhiltsov-max]

Motivation and context

Fixes #1934

Interpolated shapes were inserted with keyframe = True, so all exported tracks had keyframe=1 everywhere in CVAT for video and other formats.

• Fixed keyframe attribute of interpolated shapes
• Added tests

How has this been tested?

Unit tests

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6592

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.004%) to 64.942%

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Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	3	75.22%

Totals
Change from base Build 6575:	-0.004%
Covered Lines:	11080
Relevant Lines:	16658

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💛 - Coveralls