services: Redact account names from sync security events

[pbzin]

Summary

• redact account names from the SyncStorageEngine malformed-authority security event
• retain the account type, provider and user information needed for diagnostics

Root cause

The event concatenated an Account object directly, implicitly calling
Account.toString(). That representation includes the complete account name and
can expose email addresses in the events log buffer before the first unlock.

Use the framework's canonical Account.toSafeString() representation instead.
It masks letters and digits in the account name while preserving useful
diagnostic context.

Validation

• synchronized against the current crDroid 16.0 frameworks/base
• verified the change applies cleanly on commit 7581a03c7356
• git diff --check
• confirmed Account.toSafeString() masks account-name letters and digits via
  Account.toSafeName()

Runtime confirmation requires a build containing this change.