Refactor SecureRandom #243

Job	Run time
Analyze (cryptography_flutter/example, beta)	44s
Analyze (cryptography_flutter, 3.35.0)	1m 0s
Analyze (cryptography_flutter, beta)	47s
Analyze (cryptography_flutter/example, 3.35.0)	51s
Analyze (cryptography_flutter_integration_test, 3.35.0)	57s
Analyze (cryptography, beta)	31s
Analyze (cryptography_flutter_integration_test, beta)	1m 32s
Analyze (cryptography_test, beta)	34s
Analyze (cryptography, 3.3.0)	46s
Analyze (jwk, 3.3.0)	45s
Analyze (cryptography_test, 3.3.0)	38s
Analyze (jwk, beta)	51s
PANA (cryptography)	1m 55s
PANA (jwk)	1m 53s
Test (cryptography, dart2js)	3m 17s
Test (cryptography, vm)	46s
Test (cryptography, dart2wasm)	2m 35s
Test (jwk, dart2wasm)	37s
Test (jwk, dart2js)	35s
Test (jwk, vm)	21s
PANA (cryptography_flutter)	1m 35s
	23m 30s

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