Move Abseil to flutter/third_party/abseil-cpp (#831)

In combination with

https://flutter-review.googlesource.com/c/third_party/abseil-cpp/+/55848,
this updates Flutter's references to Abseil from
//third_party/abseil-cpp to //flutter/third_party/abseil-cpp.

Will be followed by a DEPS roll in flutter/engine and patches to our own
BUILD.gn files and #includes.

Issue: flutter/flutter#144201
Part of: flutter/flutter#67373

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build_overrides/angle.gni

@@ -18,7 +18,7 @@ if (!is_fuchsia) {
 }
 
 # Overrides for ANGLE's dependencies.
-angle_abseil_cpp_dir = "//third_party/abseil-cpp"
+angle_abseil_cpp_dir = "//flutter/third_party/abseil-cpp"
 angle_glslang_dir = "//flutter/third_party/vulkan-deps/glslang/src"
 angle_googletest_dir = "//third_party/googletest/googletest/src"