ICU-22991 Remove protected direct access to fStamp #164

Job	Run time
clang-valgrind-test	10m 13s
clang-valgrind-intltest (utility)	6m 39s
clang-valgrind-intltest (normalize)	6m 38s
clang-valgrind-intltest (collator)	5m 23s
clang-valgrind-intltest (regex)	5m 34s
clang-valgrind-intltest (format)	16m 17s
clang-valgrind-intltest (translit)	6m 10s
clang-valgrind-intltest (rbbi)	8m 15s
clang-valgrind-intltest (rbnf)	6m 25s
clang-valgrind-intltest (rbnfrt)	6m 15s
clang-valgrind-intltest (icuserv)	5m 17s
clang-valgrind-intltest (idna)	5m 33s
clang-valgrind-intltest (convert)	6m 42s
clang-valgrind-intltest (rbnfp)	5m 22s
clang-valgrind-intltest (csdet)	5m 28s
clang-valgrind-intltest (spoof)	5m 13s
clang-valgrind-intltest (bidi)	5m 50s
	1h 57m 14s

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