Add content-based colorScheme functionality (flutter#122919)

* add shadowColor property

* add to bottom app bar

* basic MCU connection demo

* more image tests

* demo update

* remove branch changes

* demo cleanup

* cleanup

* update algo

* working consistent example

* demo cleanup

* Added tests

* fix merge

* rebase

* basic MCU connection demo

* more image tests

* demo update

* remove branch changes

* demo cleanup

* cleanup

* update algo

* working consistent example

* demo cleanup

* Added tests

* lint fixes

* fix theme error

* whitespace fixes

* revert old commit

* update test formatting

* modify image source to external repo

* remove pngs

* add blank line

* fix web tests

* comment responses

* remove whitespace

* whitespace

* whitespace

Author: Eilidh Southren