Make DeprecatedOrUnsafeBufferHandling not high severity

This tidy check has been disabled by default globally,
in build/soong/cc/config/tidy.go.
Local projects can enable it to find/fix such issues,
but if not fixed, it should not be treated as a
high severity one and not to alarm daily builds.

Bug: 227361697
Test: warn.py --url=http://cs/android --separator='?l=' build.log > warnings.html
Test: warn.py --gencsv build.log > warnings.csv
Change-Id: Ifa65a788ddc6a67ed3250d5bed737c6e3b153617