Undo a breaking change on ReactViewGroup constructor (#47423)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/47423

cipolleschi found out that we broke the `ReactViewGroup` constructor when making this class Nullsafe.

Specifically now users would need to pass a `Context` and not a `Context?` as libraries will break (and this will break a lot of them).
So I'm undoing this change by annotating this parameter as Nullable.

Changelog:
[Android] [Changed] - Undo a breaking change on ReactViewGroup constructor

Reviewed By: tdn120

Differential Revision: D65483379

fbshipit-source-id: 43f7bcab5f10b1b8b5601de1dab314d7bbb1eefe