SyntheticPasswordManager: fix naming of scrypt parameters

scryptN, scryptR, and scryptP are really the log base 2 of the scrypt
parameters N, R, and P -- not N, R, and P themselves as their names
suggest.  Rename them accordingly.

No change in behavior; this is just a cleanup.

Test: atest SyntheticPasswordTests
Change-Id: I2b3bfe73e623ebf6f0f9d4de11f6e7d00d825522