Replace RxShell with `FlowShell` #2128

Job	Run time
Lint vitals (Foss, Beta, app)	7m 42s
Build apps (Foss, Debug, app)	7m 18s
Lint vitals (Foss, Release, app)	7m 20s
Unit tests (Debug, test)	6m 38s
Build apps (Gplay, Debug, app)	7m 13s
Lint vitals (Gplay, Beta, app)	7m 44s
Unit tests (Debug, testFoss)	7m 13s
Lint vitals (Gplay, Release, app)	7m 23s
Unit tests (Debug, testGplay)	7m 11s
Unit tests (Beta, test)	5m 10s
Unit tests (Beta, testFoss)	7m 18s
Unit tests (Beta, testGplay)	7m 27s
Unit tests (Release, test)	4m 51s
Unit tests (Release, testFoss)	7m 15s
Unit tests (Release, testGplay)	7m 20s
	1h 45m 3s

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