Playback with SoundPool

### Main changes
- Added 4 WAV files for testing playback
- Implemented PlayState class. Using SoundPool for loading and playing sounds

### Post-Merge Testing Results

#### Playback
- At initial app launch and first button press, test sound doesn't play because it hasn't loaded yet. Subsequent button presses play the sound correctly.
- Switching back to Play state from Record state, playback functionality still works (at least for test sound files)

#### Recording
- When you press any of the four buttons, a 3-second countdown timer appears. Couldn't record though since I'm using an emulator.