Password is only used in PBKDF2

Ensure that the secret that LockSettingsService passes us, which we
somewhat inaccurately call a "password", is used only as input to PBKDF2
by wrapping it in a Password type.

Bug: 163866361
Test: keystore2_test
Change-Id: I5eb964cb9ffe97902dfeec17c328766f79aa5646
7 files changed