Improve spectrum rendering

[supersat]

Currently, the seaglass app depends on MPAndroidChart, which produces some weird spaces between bars and probably isn't designed to be dynamically updated. It should be relatively easy to draw the spectrum ourselves using native Android APIs.

Additionally, spectrum measurements are currently pulled from the cell_log fifo, which means that they are only updated once the spectrum has been swept and not in real time. However, we can parse the layer1 console output and generate spectrum-measurement events in real-time, allowing us to draw the spectrum as it's being measured.

Ideally, we'd also highlight the current ARFCN being scanned during the cell sync phase.