Work towards https://github.com/flutter/flutter/issues/132712.
After this PR, after a completed `flutter build apk` command, we:
- Emit a `manifest-impeller-disabled` command if `io.flutter.embedding.android.EnableImpeller` is `'false'`.
- Emit a `manifest-impeller-disabled` command if `io.flutter.embedding.android.EnableImpeller` is _missing_.
- Emit a `manifest-impeller-enabled` command if `io.flutter.embedding.android.EnableImpeller` is `'true'`.
We will need to change the default (see `_impellerEnabledByDefault` in `project.dart`) before releasing, otherwise we will misreport `manifest-impeller-disabled` at a much higher rate than actual. If there is a way to instead compute the default instead of hard-coding, that would have been good.
See <https://docs.flutter.dev/perf/impeller#android> for details on the key-value pair.
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I also did a tad of TLC, by removing the (now-defunct) `Usage` events for `flutter build ios`, so they are consistent.
/cc @zanderso, @chinmaygarde, @jonahwilliams
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