keystore: rename the PBKDF2 functions

Rename Password::derive_key() to Password::derive_key_pbkdf2(), and
rename generateKeyFromPassword() to PBKDF2().  This helps distinguish
these functions from the HKDF functions, including the existing ones as
well as the Password::derive_key_hkdf() added by the next CL.

Bug: 296464083
Bug: 314391626
Test: atest -p --include-subdirs system/security/keystore2
Change-Id: I76e5ee5a5c6452951727be6fce1a43a2322a3950