This updated the native embedder API in a non-ABI breaking way.

With the update to HEAD of the Fuchsia buildtools repo, the new clang
toolchain picked up caused link-time breakage in android x86_64
libFlutter.so builds.

Sample log:
https://build.chromium.org/p/client.flutter/builders/Linux%20Engine/builds/1974/steps/build%20android_debug_x64/logs/stdio

Sample failure:
FAILED: libflutter.so libflutter.so.TOC lib.stripped/libflutter.so
../../third_party/android_tools/ndk/toolchains/x86_64-4.9/prebuilt/linux-x86_64/lib/gcc/x86_64-linux-android/4.9.x/../../../../x86_64-linux-android/bin/ld.gold: error: obj/flutter/shell/platform/android/libflutter/android_context_gl.o: unsupported reloc 42 against global symbol std::__ndk1::num_put<char, std::__ndk1::ostreambuf_iterator<char, std::__ndk1::char_traits<char> > >::id

This reverts commit 8ad42f0d.

* Roll Fuchsia buildtools to 85907c59e97527d79bbfdfd849d8e85c24959cc5

This also updates Flutter buildroot to a6e52dbb776c45cc8c57d7143b8eb8b2e762fdfb
which disables -Wtautological-constant-compare temporarily until
https://reviews.llvm.org/D39462 lands in clang. This is in line with
Fuchsia's compiler options.

* Apply clang-format diffs

No logical changes. This applies clang-format from the latest Fuchsia
buildtools to the engine codebase.

* The Flutter engine will be shipped as a shared library.
* The engine is renderer and window toolkit agnostic.
* The simple public C API is described in embedder.h.
* ABI breaking changes will be indicated by changing the FLUTTER_ENGINE_VERSION.
* A simple GLFW based example of this API is available at https://gist.github.com/chinmaygarde/8abf44921f7d87f6da7bf026267c4792