Update (android_alarm_manager_plus): Clean up plugin code, fix potential issue, improve README

[vbuberen]

Description

This PR does a clean up in plugin code mainly based on lint reported issues.
Additionally one of updated parts might fix potential issue with adding persistent alarms, since shared preferences modification was done in a wrong way (see the attached screenshot)

Finally, updated README for the plugin:
1). Removed mentions of different embedding versions, because there is no V1 used in the plugin anymore as well as the lower version of Flutter is 1.12, so there is no need to describe anything for projects with older versions.
2). Added info about WidgetsFlutterBinding.ensureInitialized() to example of usage to address multiple reported issues.

Related Issues

Readme updates close #446 and partially #276, because 276 refers to more plugins, which also need the same mention.

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]

LGTM, there's just a java formatting issue that needs to be fixed before merging. See more info here: https://github.com/fluttercommunity/plus_plugins/pull/517/checks?check_run_id=3808532675