SF: Make thread safety macros zero-cost

ON_MAIN_THREAD(mDisplays) incurred a copy. In other words, this failed:

    decltype(auto) displays = ON_MAIN_THREAD(mDisplays);
    static_assert(std::is_lvalue_reference_v<decltype(displays)>);

Bug: 182939859
Test: static_assert
Test: simpleperf
Change-Id: I19900d0350b832cd71457e5a6f11fa65b16e0412