Return handle to gatekeeper password instead of actual password

This allows 1) LockSettingsService to ensure the password isn't
valid forever. 2) Password to stay in system_server

Also, adds ILockSettings#removeGatekeeperPasswordHandle
so that callers can clean up after themselves.

Bug: 161765592

Run the following on face/fingerprint devices
Test: Remove credential
      adb shell am start -a android.app.action.SET_NEW_PASSWORD
      Set up credential + fingerprint
Test: Remove credential,
      adb shell am start -a android.settings.FINGERPRINT_SETTINGS
      This tests the ChooseLock* logic in FingerprintSettings
Test: Set up credential,
      adb shell am start -a android.settings.FINGERPRINT_SETTINGS
      This tests the ConfirmLock* logic in FingerprintSettings
Test: Remove device credential, enroll fingerprint/face. Succeeds.
      This tests the ChooseLock* returning SP path from
      BiometricEnrollIntro
Test: With credential and fingerprint/face enrolled, go to
      fingerprint/face settings and enroll. This tests the
      ConfirmLock* path in Fingerprint/FaceSettings
Test: Remove device credential, enroll credential-only, enroll
      fingerprint/face separately. Succeeds. This tests the
      ConfirmLock* returning SP path in BiometricEnrollIntro
Test: In SUW, set up credential, then biometric. This tests
      the ChooseLock* path in SUW
Test: In SUW, set up credential, go back, then set up biometric.
      This tests the ConfirmLock* path in SUW

Change-Id: I76534acc7af1e0f2e4d8af369fee31d1e19ddba9