Close keyboard on hardware back press

[millerovv]

Description

• Aligns behavior of the keyboard with native Android keyboard and closes it when user pressed hardware "back".
• Adds Android project to the math_keyboard_demo.
• Main changes are in math_field.dart.

https://github.com/simpleclub/math_keyboard/assets/31796433/937f0434-4034-465a-9756-8f821017b649

Related issues & PRs

https://simpleclub.atlassian.net/browse/SC-10050

Checklist

Remove If [...] items that do not apply to your PR.

• [x] I have made myself familiar with the CaTeX contributing guide.
• [x] I added a PR description.
• [x] I linked all related issues and PRs I could find (no links if there are none).
• [x] If this PR changes anything about the main math_keyboard or example package (also README etc.), I created an entry in CHANGELOG.md (## UPCOMING RELEASE if the change on its own is not worth an update).
• [x] If this PR includes a notable change in the math_keyboard package, I updated the version according to Dart's semantic versioning.
• [x] If there is new functionality in code, I added tests covering all my additions.
• [x] All required checks pass.

[tomkuschka]

@millerovv The dismiss animation is not triggered when I hardware back is pressed ☹️ Since the animation happens on when the return key is pressed, I think we should also play it here

[millerovv]

@tomkuschka The closing animation is now working:

https://github.com/simpleclub/math_keyboard/assets/31796433/5e1b466d-7794-4986-806f-4c08c4e39304

[codecov-commenter]

Codecov Report

Merging #56 (7180251) into main (f586c6b) will decrease coverage by 0.34%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
- Coverage   27.44%   27.11%   -0.34%     
==========================================
  Files          11       11              
  Lines        1133     1147      +14     
==========================================
  Hits          311      311              
- Misses        822      836      +14     

Flag	Coverage Δ
math_keyboard	27.11% <0.00%> (-0.34%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
math_keyboard/lib/src/widgets/math_field.dart	0.00% <0.00%> (ø)