[sf] Stop inheriting from std::unary_function

std::unary_function was deprecated in C++11 and removed in C++17. Its
function is to provide two typedefs, `argument_type` and `result_type`,
but these are not needed for std::find_if's Predicate, so stop using
std::unary_function.

Bug: 175635923
Test: m MODULES-IN-frameworks-native
Change-Id: Ie16ada6f76bee7d9f248dc8349a095b50777c92d
Merged-In: Ie16ada6f76bee7d9f248dc8349a095b50777c92d