Improve the LockSettingsService class comment

import javax.crypto.spec.GCMParameterSpec;

/**

 * Keeps the lock pattern/password data and related settings for each user. Used by

 * LockPatternUtils. Needs to be a service because Settings app also needs to be able to save

 * lockscreen information for secondary users.

 * LockSettingsService (LSS) mainly has the following responsibilities:

 * <p>

 * <ul>

 *   <li>Provide APIs to verify and change the Lock Screen Knowledge Factor (LSKF) ("lockscreen

 *   credential") of each user.  Unlock users when their correct LSKF is given.</li>

 *

 *   <li>Store other lockscreen related settings, such as some Keyguard (UI) settings.</li>

 *

 *   <li>Manage each user's synthetic password (SP), which is their main cryptographic secret.

 *   See {@link SyntheticPasswordManager}.</li>

 *

 *   <li>Protect each user's SP using their LSKF.  Use the Gatekeeper or Weaver HAL to ensure that

 *   guesses of the LSKF are ratelimited by the TEE or secure element.</li>

 *

 *   <li>Protect each user's data using their SP.  For example, use the SP to encrypt/decrypt the

 *   user's credential-encrypted (CE) key for file-based encryption (FBE).</li>

 *

 *   <li>Generate, protect, and use profile passwords for managed profiles.</li>

 *

 *   <li>Support unlocking the SP by alternative means: resume-on-reboot (reboot escrow) for easier

 *   OTA updates, and escrow tokens when set up by the Device Policy Controller (DPC).</li>

 *

 *   <li>Implement part of the Factory Reset Protection (FRP) and Repair Mode features by storing

 *   the information needed to verify a user's LSKF on the persist or metadata partition.</li>

 *

 *   <li>Support insider attack resistance using the AuthSecret HAL.</li>

 *

 *   <li>Implement "recoverable keystore", a feature that enables end-to-end encrypted backups.

 *   See {@link android.security.keystore.recovery.RecoveryController}.</li>

 * </ul>

 * <p>

 * The main clients of LockSettingsService are Keyguard (i.e. the lockscreen UI, which is part of

 * System UI), the Settings app (com.android.settings), and other parts of system_server.  Most

 * methods are protected by ACCESS_KEYGUARD_SECURE_STORAGE which only system processes can have.

 *

 * @hide

 */