Change RoIImage and MosaicImage to have np.uint8 dtype as default

[vinnamkim]

Summary

• This is something missed in Change image default dtype from float32 to uint8 #1175

How to test

I believe the existing tests (tests/unit/test_images.py) can cover it.

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

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) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f0d7b13) 80.53% compared to head (7a3a4f1) 80.53%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1245   +/-   ##
========================================
  Coverage    80.53%   80.53%           
========================================
  Files          270      270           
  Lines        30243    30241    -2     
  Branches      5905     5904    -1     
========================================
- Hits         24356    24355    -1     
  Misses        4506     4506           
+ Partials      1381     1380    -1     

Flag	Coverage Δ
ubuntu-20.04_Python-3.8	80.51% <100.00%> (+<0.01%)	⬆️
windows-2022_Python-3.8	80.50% <100.00%> (+<0.01%)	⬆️

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