signin: add cross-platform biometric auth flow #2039

Job	Run time
Build (Unit-tests & Android-Lint)	10s
Android UI-tests on emulator (26, default)	9s
Android UI-tests on emulator (23, default)	11s
Android UI-tests on emulator (28, default)	9s
Android UI-tests on emulator (30, default)	9s
Android UI-tests on emulator (36, default)	3m 28s
Android UI-tests on emulator (33, default)	7s
Android UI-tests on emulator (35, default)	3m 28s
Android UI-tests on emulator (25, default)	3m 28s
Android UI-tests on emulator (31, default)	3m 28s
Android UI-tests on emulator (29, default)	3m 28s
Android UI-tests on emulator (32, default)	9s
Android UI-tests on emulator (27, default)	3m 28s
Android UI-tests on emulator (24, default)	3m 28s
Android UI-tests on emulator (34, default)	3m 28s
	28m 48s

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