Add two channel and manage them for push notif

[HugoGresse]

• add two new channel id (and change the android default one) in order to have one channel with sound and very important, the other less important without sound.
• change notification push type if the room is starting in less than 5 min. If it's more, it a reminder push notif, otherweise it's a sound notif with "URGENCY".
• manage channel id in functions