Add MessageChannel and tests #6386

Job	Run time
raspi-2 / initialize	32s
raspi-2-skia / initialize	33s
raspi-2 / docker-unittest-image	0s
raspi-2 / docker-build-image	1m 12s
raspi-2-skia / docker-unittest-image	0s
raspi-2-skia / docker-build-image	2m 27s
raspi-2 / raspi_devel	8m 23s
raspi-2 / raspi_debug	10m 24s
raspi-2 / raspi_qa	7m 36s
raspi-2 / raspi_gold	6m 23s
raspi-2 / sbversion-15_devel	7m 17s
raspi-2 / sbversion-15_debug	11m 27s
raspi-2 / sbversion-15_qa	7m 14s
raspi-2 / sbversion-15_gold	7m 26s
raspi-2-skia / raspi-2-skia_devel	7m 15s
raspi-2-skia / raspi-2-skia_debug	10m 14s
raspi-2-skia / raspi-2-skia_qa	6m 43s
raspi-2-skia / raspi-2-skia_gold	6m 36s
raspi-2 / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2 / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
raspi-2-skia / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2-skia / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
	1h 41m 42s

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