This commit doesn't resolve all the violations. Specifically,
INDEX_UNSET is still being incorrectly passed as various IntDef values
to the PlayerInfo constructor, but making these parameters (and the
corresponding PlayerInfo fields) @Nullable is a more involved change
(and it's not obvious at what point we can guarantee the value will be
non-null/non-INDEX_UNSET), so it's left for a later commit.
PiperOrigin-RevId: 407294091
