* Add support for image insertion on Android

* Fix checks

* Use proper Dart syntax on snippet

* Specify type annotation on list

* Fix nits, add some asserts, and improve example code

* Add missing import

* Fix nullsafety error

* Fix nullsafety error

* Remove reference to contentCommitMimeTypes in docs

* Fix nits

* Fix warnings and import

* Add test for content commit in editable_text_test.dart

* Check that URIs are equal in test

* Fix nits and rename functions / classes to be more self-explanatory

* Fix failing debugFillProperties tests

* Add empty implementation to `insertContent` in TextInputClient

* Tweak documentation slightly

* Improve docs for contentInsertionMimeTypes and fix assert

* Rework contentInsertionMimeType asserts

* Add test for onContentInserted example

* Switch implementation to a configuration class for more granularity in setting mime types

* Fix nits

* Improve docs and fix doc tests

* Fix more nits (LongCatIsLooong)

* Fix failing tests

* Make parameters (guaranteed by platform to be non-nullable) non-nullable

* Fix analysis issues