feat(android_alarm_manager_plus) Handle Android 12 behavior changes

[vbuberen]

Description

Android 12 introduced some behavior changes for scheduling exact alarms. Also, Alarm manager plugin recently had some internal updates to target Android 12, but no changes to the AlarmService logic were done.

This PR updates the plugin to check if app still has permission to schedule exact alarms before trying to schedule one, so users won't see SecurityException.
Additionally, I did some updates to plugin documentation as it seems quite outdated due to recent changes.

Related Issues

No related issues so far due to not many apps switching to target SDK 31, I believe.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I am willing to follow-up on review comments in a timely manner.

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 a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[miquelbeltran]

Thanks a lot for your awesome contribution!