Fix alarm_manager_plus for Android 12+

[ThexXTURBOXx]

Description

This PR fixes android_alarm_manager_plus for Android 12+ by adding the FLAG_IMMUTABLE to the PendingIntents.

Related Issues

Fixes #477

Checklist

• 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

• 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.

[mhadaily]

Seems formatting is failing.

Could you please add a changelog and bump the package as well?

thanks.

[ThexXTURBOXx]

Yes, give me a few minutes. I will fix it asap! :)

[ThexXTURBOXx]

@mhadaily The issues should be fixed now :)

[mhadaily]

LGTM

[SalishKumar]

please fix this

[ThexXTURBOXx]

@SalishKumar There is already a PR for this. Apologies for the mistakes, I am almost certain I just forgot to git add * again