Always reset the LAContext if we're trying to force a biometric prompt

[jhaven-stytch]

1. iOS doesn't enforce biometrics on reading from the keychain, only on writing to it
2. As a result, we have to explicitly request authentication when reading from the keychain in the desired areas
3. iOS reuses the authentication context within a session (to prevent "authentication fatigue" for users), which prevents the policy evaluation from running
4. So we have to both explicitly request authentication AND explicitly request a new authentication context if we want to ensure that the policy evaluation occurs

Changes:

Checklist:

• I have verified that this change works in the relevant demo app, or N/A
• I have added or updated any tests relevant to this change, or N/A
• I have updated any relevant README files for this change, or N/A