Gavin/add metronome

[gavination]

Adds a properly functioning metronome! State transitions still need work, but this PR addresses the following:

• Adds a circular buffer to provide a rolling window for FFT analysis
• Revises some of the transition logic for better readability
• Adds app icons for Android
• Adds app icons for iOS
• Modifies detection threshold for determining tremor activity

[gavination]

Shoutouts to @LordTocs for the huge assist on this one.