C++17 compatibility: add missing `constexpr`.

Fixes this error when we move the global default to C++17:

  hardware/google/av/codec2/tests/C2_test.cpp:142:19: error: static_assert expression is not an integral constant expression
    static_assert(c2_cntr32_t(1) ==
c2_cntr32_t(c2_cntr64_t(0x100000001ul)), "1 == 1");
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Bug: http://b/111067277
Test: builds
Change-Id: I66b8573d5dbdb02f953ec54b77eff274333c6f83