feat: replace Platform.isX with defaultTargetPlatform

[Gustl22]

Description

Flutter recommends using defaultTargetPlatform instead of Platform.isX, where possible. This also enables testing platform specific dart code in unit tests (but without native code), which is way more convenient and faster.

Checklist

• The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, docs:, chore: etc).
• I have read the [Contributor Guide] and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in [example].

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

[spydon]

The dart:io.Platform object should only be used directly when it's critical to actually know the current platform, without any overrides possible (for example, when a system API is about to be called).

Isn't that the case here?

[Gustl22]

Isn't that the case here?

Where exactly do you mean? Building the AndroidContext or converting to json doesn't call any native platform methods. So it's not critical in my opinion. Further we cannot mock the platform to test these methods without an integration test, which should not be needed in many cases. See this test which has different outcomes for different platforms, but is not needed to be executed on an according device / emulator.

[spydon]

Isn't that the case here?

Where exactly do you mean? Building the AndroidContext or converting to json doesn't call any native platform methods. So it's not critical in my opinion. Further we cannot mock the platform to test these methods without an integration test, which should not be needed in many cases. See this test which has different outcomes for different platforms, but is not needed to be executed on an according device / emulator.

Alright, sounds good then. :)