Modernise GifTranscoder JNI files

[mikomikotaishi]

This PR addresses the following in the native JNI parts of GifTranscoder:

• Set an explicit C++ version (C++17), which is the NDK default
• Replace macros with constexpr functions and constants where possible
• Add [[nodiscard]] attributes, to explicitly highlight when a function call's return goes unused
• Add noexcept specifiers
• Replace C-style castes (T) with static_cast<T> and reinterpret_cast<T>
• Replace NULL with nullptr
• Replace a typedef with a using alias
• Replace bodyless constructor/destructor with = default
• Replace int[4] with std::array<int, 4>
• Replace the C ::timespec and ::clock_gettime() with std::chrono::steady_clock
• Explicitly disable copy constructor and copy assignment operators on GifFilesCloser

[mikomikotaishi]

@m4pl @RankoR @thestinger I'd like to request a PR review (I do not have access to assign reviewers)

[RankoR]

Thanks for your contribution.

It's good in general, but could you please do these adjustments?

1. Update the PR description and commit message to explicitly mention the x86_64 signed-char undefined behavior and color-corruption fix
2. Add a regression test using color channels above 0x7f. An end-to-end test that transcodes the small 4×2 fixture and checks the resulting two pixels would catch the old implementation
3. Keep the functional fix in a separate commit from the mechanical modernization

[mikomikotaishi]

Where would you like the test to go?

[RankoR]

@mikomikotaishi you can add something like app/src/androidTest/java/com/android/messaging/util/GifTranscoderTest.kt, put a GIF in app/src/androidTest/assets/gif_transcoder/high_color_channels_4x2.gif and load it from the test.