Support negation for all boolean engine flags from the command line #193354

Currently, presence-only boolean engine flags (such as --enable-software-rendering, --trace-startup, and --verbose-logging) cannot be overridden or disabled from the command line if enabled via platform metadata (such as AndroidManifest.xml) unlike flags with values can (like --enable-impeller=true|false). Because these switches lack a negation syntax (e.g., --flag=false or --no-<flag>), the engine only checks for presence via fml::CommandLine::HasOption(), creating an asymmetric precedence model where command-line flags cannot override manifest settings without manually modifying source configuration and rebuilding.

Because of this, we should consider supporting negation for presence-only boolean engine flags so that the precedence between command line and manifest defined flags is consistent.

Possible approaches

1. Standardize on =true|false switches: Transition all boolean engine flags to key-value toggles (similar to --enable-impeller=true|false), allowing command-line arguments to override manifest settings using standard last-argument-wins evaluation.
2. Support --no-<flag> cancellation: Introduce --no-<flag> syntax support in the engine switch parser or embedder loaders (e.g., FlutterLoader) that explicitly cancels out flags previously injected by platform metadata.