Issue template: Replace "host machine OS" by text appropriate for mobile devices

[gerritholl]

In the Github issue template, replace the "host machine OS" description to be more appropriate for mobile devices. Instead of mentioning Linux or Windows, ask the user to tell their Mobile device and Android version

Checklist

• Test your changes as thoroughly as possible before you commit them. Preferably, automate your test by unit/integration tests.
• If this PR comes from a fork, please Allow edits from maintainers
• Set a speaking title. Format: {task_name} (closes #{issue_number}). For example: Use logger (closes # 41)
• Link your Pull Request to an issue (if applicable)
• Create Work In Progress [WIP] pull requests only if you need clarification or an explicit review before you can continue your work item.
• Make sure that your PR is not introducing unnecessary reformatting (e.g., introduced by on-save hooks in your IDE)
• Make sure that your PR does not contain changes in strings.xml (see issue [HELP WANTED][README] Text / Translations  #332)
• Make sure that your PR does not contain compiled sources (already set by the default .gitignore) and / or binary files

Description

The github issue template for host machine OS was confusing and not relevant for mobile users, which means all users of cwa-app-android. Instead, ask for android version and smartphone model.