Previously, using Platform.executable (from dart:io) returned null (if
non-null-by-default was disabled) or threw an exception (if NNBD was
enabled) since we weren't setting it.

We now pass the executable name to Dart during VM startup based on the
first value in the FlutterProjectArgs::command_line_argv array passed to
FlutterEngineRun (or FlutterEngineInitialize) on startup. argv[0] (if
specified) is explicitly documented as being required to be the
executable name in embedder.h. In the case where no argv[0] is
specified, we instead set Platform.executable to "Flutter" in order to
avoid violating the (non-nullable) type annotation on
Platform.executable.

Note that dart::bin::SetExecutableName() does NOT make a copy of the
input string, so that value needs to be available for the entire lifetime
of the VM.

This also adds EmbedderConfigBuilder::SetExecutableName() to support
setting a fake executable name in unittests. By default, we continue to
set the name "embedder_unittest" unless overridden using this method.

See: https://api.flutter.dev/flutter/dart-io/Platform/executable.html
See: dart-lang/sdk#48427

Issue: flutter/flutter#83921

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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