Warn on updating the deprecated serial blocklist

Move the start of the thread to onChange to reduce the indentation
level of writeDenylist.

Remove unnecessary comments.

Test: atest CtsSecurityTestCases:android.security.cts.CertBlocklistFileTest
Test: adb shell cmd settings put --user 0 secure serial_blacklist 1234;
      manually check logs
Bug: 340363351
Change-Id: I1d74c8c6899fe19701de2d2788f83327ff6bd2e0