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

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] A purchase does not increase your chances of winning. Void in
Quebec. Void where prohibited or restricted by law.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
/~https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: /~https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
/~https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
/~https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[flutter/tests]: /~https://github.com/flutter/tests
[breaking change policy]:
/~https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: /~https://github.com/flutter/flutter/wiki/Chat

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"