SyntheticPasswordManager: rename derivePassword() to deriveSubkey()

The output of derivePassword() is a key (byte array, not hex encoded),
not a password.  Therefore, rename it to deriveSubkey().

No change in behavior; this is just a cleanup.

Test: atest SyntheticPasswordTests
Change-Id: Ie3d74a579963bcbe63fc93c1b4c436c2aba8c342
1 file changed