RFR: 8293858: Change PKCS7 code to use default SecureRandom impl instead of SHA1PRNG

[Sean Mullan]

For timestamp nonces, change the PKCS7 code to use the default SecureRandom PRNG (which varies depending on the OS), instead of SHA1PRNG.

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Commit messages:
 - Change PKCS7 code to use default SecureRandom impl instead of SHA1PRNG.

Changes: https://git.openjdk.org/jdk/pull/10883/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10883&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293858
  Stats: 23 lines in 1 file changed: 1 ins; 19 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10883.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10883/head:pull/10883

PR: https://git.openjdk.org/jdk/pull/10883