Fix notification while on silent

[Felixre7]

Fix Android rest notification with playback through active headphones and a generated tone sequence. The tones were the easiest thing I could do to try and be universal but i don't know if this really goes with the vibe of the app. If you don't want to take this there is a simpler version in which you just use the normal notification sound when the timer ends. Also please review the tones i used. There were just a quick solution but some other dial tone frequency might sound nicer to you. These are a little grating, but they get your attention!

• Opt in for the 3 second count down and obey the main notification slider
• Generate a single PCM buffer for the DTMF 4-4-4-6 sequence: 500 ms countdown tones and a one-second final tone.
  - doing them independently caused some weird timing issues
• Preserve notification deadlines and clean up playback on completion, cancellation, and audio route/focus changes.

The feature is Android-only and tested for 1 workout session.
This PR does not add iOS countdown audio nor is it tested.

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