Do not store hashed password on device

 - Store device salt with AccountManager
 - Store hmac secret with AccountManager
 - Refresh public key signatures on login
 - Do not store hashed password
 - Do not attempt to reauthenticate if refresh token is invalid
 - Prompt user for reauthentication if refresh token is invalid

Change-Id: I77cfeb7d51976282a0a890c54545d9bde2382e51