Mic icons not themed in indicator menu

[cibersheep]

1- Connect headphones with microphone
2- Pull down sound indicator

Expected: themed icon mute mic + slider + max volume mic themed icon

Current: fall back mic icon + slider + fall back mic icon

Observations:

• This might be the responsible for the mic slider: https://github.com/ubports/indicator-sound/blob/xenial/src/sound-menu.vala#L123
• One themed icon exists in the image.
• Looks like the both icons ("audio-input-microphone-low-zero-panel" and "audio-input-microphone-high-panel") falls back to /usr/share/icons/Humanity/devices/24/audio-input-microphone.svg
• Renaming /usr/share/icons/suru/status/scalable/audio-input-microphone-low-zero-panel.svg as audio-input-microphone.svg makes both icons in the slider to look themed

[mateosalta]

we have both audio-input-microphone-symbolic and audio-input-microphone-muted-symbolic, and will give us a low and high option of icons.
#19

[cibersheep]

I thought it should have to work with the plugin like it is before your MR though