test(device_info_plus): Add Android display metrics check, fix Android version check

[vbuberen]

Description

After adding an Android build matrix saw that integration tests fail for device_info_plus on newly added Andoid APIs. As it turned out there was a hardcoded version of Android (30), which caused test to fail on all other versions.
Also, saw that same test was missing check for a newly added device metrics data, so added it as well.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[nohli]

LGTM!

[vbuberen]

Test was still failing on API 21 because some properties are not available till API 23:
https://developer.android.com/reference/android/os/Build.VERSION#BASE_OS
https://developer.android.com/reference/android/os/Build.VERSION#PREVIEW_SDK_INT
https://developer.android.com/reference/android/os/Build.VERSION#SECURITY_PATCH

And the implementation had the check for API version, thus, returning null on lower versions:
https://github.com/fluttercommunity/plus_plugins/blob/main/packages/device_info_plus/device_info_plus/android/src/main/java/dev/fluttercommunity/plus/device_info/MethodCallHandlerImpl.java#L73

Fixed it as well.

[nohli]

Is it somewhere in our docs that they return null on lower versions?

[vbuberen]

Is it somewhere in our docs that they return null on lower versions?

No. we don't have it. I will create a separate PR as I see one more possible improvement for our docs on the website for this plugin.

[nohli]

If even we don't really know, we should put it in the docs of the methods that return null for specific versions.