Fix notifications not being shown on modern Androids

[di72nn]

Hi!

Since targetSdkVersion 26 notifications without a notification channel are not shown on Android 8+ (see this).

This PR adds two notification channels: sync for silent sync notifications and noisy default for everything else.

[di72nn]

@nerzhul Just a friendly reminder that to have notifications working on modern Androids you need to merge this (or implement a fix yourself).

Let me know if you want me to improve the PR.