Refactor SecureRandom #241

Job	Run time
Analyze (cryptography_flutter, 3.35.0)	1m 38s
Analyze (cryptography_test, beta)	29s
Analyze (jwk, 3.3.0)	44s
Analyze (cryptography_test, 3.3.0)	40s
Analyze (jwk, beta)	30s
Analyze (cryptography_flutter_integration_test, 3.35.0)	51s
Analyze (cryptography, beta)	31s
Analyze (cryptography_flutter/example, 3.35.0)	1m 38s
Analyze (cryptography_flutter, beta)	1m 38s
Analyze (cryptography_flutter_integration_test, beta)	51s
Analyze (cryptography_flutter/example, beta)	41s
Analyze (cryptography, 3.3.0)	52s
PANA (cryptography)	1m 55s
Test (cryptography, dart2js)	4m 4s
Test (cryptography, vm)	49s
Test (jwk, vm)	1m 4s
Test (jwk, dart2js)	34s
Test (cryptography, dart2wasm)	2m 35s
Test (jwk, dart2wasm)	50s
PANA (jwk)	2m 3s
PANA (cryptography_flutter)	2m 29s
	27m 26s

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