This patch applies the following changes:

* Rename a stray .cc file to .mm (connection_collection.cc).
* Migrate Objective-C #includes to #import as per the style guide.
* #include/#import order updated to reflect the style guide order:
  associated header, system/standard library headers, library headers,
  platform-specific includes.
* Include cstring where we're using strlen.
* Add a missing copyright header in SemanticsObjectTest.mm.

Bugs: https://github.com/flutter/flutter/issues/60025