Use `UnreleasedFlag` instead of `SysPropBooleanFlag`.

A `UnreleasedFlag` is safer as it can not be changed in released builds or with public APIs (i.e., System Properties).

Test: manual

Fixes: b/257976997

Change-Id: I17eddcc77a14b16b31f67e5675932f4690b9ffb8
1 file changed