Migrate to AndroidX

[daniele-athome]

It had to be done sooner or later.

https://developer.android.com/jetpack/androidx/migrate

Also consider this comment in #1198:

Using WorkManager is a no-go for now because we're not ready to switch to AndroidX yet. However, we could just use a foreground service on API levels that require it and stick to the background service otherwise.

Known issues found so far:

• background selector in recycler view items doesn't work any more (conversations)
• migrate to recyclerview-selection (could solve problem above) and remove recyclerview-multiselect (currently jetified)
• test multiselect on tablet