[engine] Replace fml::Status and fml::StatusOr with absl - #194161

Master replaced fml::Status with absl (flutter#194161).

)

This started as a fix for `fml::Status` holding its message as a
`std::string_view`: `CommandQueueVK::Submit` built a status from a local
string inside the fence callback and returned it after the string was
gone. In review the ask was to use `absl::Status` instead, so this PR
removes `fml::Status` and `fml::StatusOr` and switches every user to
`absl::Status` and `absl::StatusOr`. `absl::Status` owns its message, so
the dangling view goes away with the type, and so does the TODO in
`status_or.h` that pointed at flutter#134741.

Most of it is mechanical. `fml::Status()` becomes `absl::OkStatus()`,
the status codes have the same names in both, includes point at
`third_party/abseil-cpp/absl/status`, and `//flutter/flow` gets abseil
status in `public_deps` because `frame_timings.h` returns it.

The part that isn't mechanical comes from `absl::Status` being
`[[nodiscard]]`. `fml::Status` wasn't, and about 25 call sites dropped a
status without looking at it. I didn't want this PR to change what any
of them do, so in production code and in the render and playground tests
they now call `.IgnoreError()`. That includes
`DescriptorPoolVK::AllocateDescriptorSets`, which ignores a failed
`CreateNewPool` today. Whether it should return that error is a separate
question. The fence waiter, descriptor pool and command encoder tests
check the result instead, since there the expected status is known.

One exception. `CopyImageByteData` in
`lib/ui/painting/image_encoding.cc` built an error for a failed swizzle
surface, never returned it, and went on to use the null surface. The
compiler only caught it because of `[[nodiscard]]`, and I added the
missing `return`. If you'd rather have that as its own PR I'll split it
out.

**Where it was verified.** macOS arm64, `host_debug_unopt_arm64`,
rebased on current main, `gn check` clean. `fml_unittests` 203/203,
`ui_unittests` 73/73, `shell_unittests` 197/197, `flow_unittests`
313/313 and the Vulkan, fence waiter, descriptor pool, command queue,
Gaussian blur and text frame tests in `impeller_unittests` (193/193).
`impeller_golden_tests` and `flutter_tester` build. Locally
`PerformanceOverlayLayerDefault.Gold` and two
`EmbedderTest.CanRenderTextWithImpeller*Metal` tests fail on missing
glyphs and ICU data (`U_MISSING_RESOURCE_ERROR`). They fail the same way
on main without this change, so it's my checkout. CI covers the other
platforms.

Fixes flutter#194151

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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