fix(messaging, android): use only one FlutterLoader instance

[russellwheatley]

Description

This method stops these warning logs from popping up when running the app:

W/FlutterJNI( 5559): FlutterJNI.loadLibrary called more than once
W/FlutterJNI( 5559): FlutterJNI.prefetchDefaultFontManager called more than once

This method was used as a fix here for this very reason.

Also used by local flutter notifications

Even in the Flutter engine itself.

Related Issues

fixes #9964

Checklist

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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.