flutter_local_notifications-v15.1.1

• [Android] fixed issue an issue similar to 2033 that was addressed in 15.0.1 where notifications on scheduled using older version of the plugin via the periodicallyShow() method would fail to have the next subsequent ones scheduled. This issue started occuring in 14.0 where support for inexact notifications was added using the ScheduleMode enum that was added and resulted in the deprecation of androidAllowWhileIdle. A mechanism was added to help "migrate" old notifications that had androidAllowWhileIdle specified but didn't account for how there are recurring notifications that were scheduled using older versions of the plugin prior to androidAllowWhile being added. This hotfix has been taken from the 16.0.0-dev.2 prerelease and has also been applied to the 14.1.3 hotfix release as well
• Updated example app so that the Android side specifies minimum SDK version version that aligns with what's specified by the Flutter SDK. This has been taken from the 16.0.0-dev.2 prerelease to allow the example app to build using recent versions where the minimum Android SDK version has changed from 16 to 19