Feature/workmanager

See issue for details: #153

• Replaces background scheduling implementation. We now use android workmanager
• Removes all the boilerplate code with syncadapter.
• Introduces option to schedule an intent with alarm manager which can skip doze if wanted.
  • This will issue a new intent which wakes up syncopoli so that workmanager can schedule a pending sync request. I found this approach non-intrusive and allows frequent sync on newer versions of android which have more restrictive doze settings.
• Introduce additional notification channel. We now have 3 channels which can be individually disabled if certain notifications are not desired: sync in progress, success, error
• Android workmanager no longer supports parables, so we add a gson dependency to serialize payload to json which we can pass as string argument.
• Add BOOT_COMPLETED intent so that syncopoli is loaded on restart of android.

I separated this work into two logical features

• workmanger (this)
• UI bugfixing (!24)

Let me know what you think.