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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
That said, I've used Flutter extensively, and can strongly recommend (IMO it's better for Android apps than native Android in many ways). There's also flutter_rust_bridge which I've not used, but heard good things about.
I've been using egui / android-activity for crusader. Its state is a bit rough lacking proper keyboard support and needing git repos. Having the GUI in Rust does help reduce FFI. egui also doesn't fit the energy efficiency requirement of OP.
NOTE:
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Hence, a higher number means a more popular project.