Create SecureRandomUtils with static SecureRandom

No need for the cost and complexity of creating an new RNG every time
we need randomness, just set up a single static one. Use the same
one in LockSettingsService and SyntheticPasswordManager.

Bug: 258560859
Test: atest LockSettingsServiceTests SyntheticPasswordTests
Change-Id: I2025f24c4710c246f661a6329a688c928ce2377d