Show a notification at sync when ContentProvider inaccessible

[sunkup]

When the contacts or calendar storage apps are disabled or missing, we show a warning in the accounts view (See #1228). We should also show a notification when the user tries to sync while their contacts or calendar storage are disabled/missing. At the moment we only log it and mark it as contentProviderError in the sync result.

See:

• davx5-ose/app/src/main/kotlin/at/bitfire/davdroid/sync/Syncer.kt

  Line 268 in 1cc9b4b

  syncResult.contentProviderError = true

• davx5-ose/app/src/main/kotlin/at/bitfire/davdroid/sync/JtxSyncer.kt

  Line 51 in 1cc9b4b

  syncResult.contentProviderError = true

• davx5-ose/app/src/main/kotlin/at/bitfire/davdroid/sync/TaskSyncer.kt

  Line 52 in 1cc9b4b

  syncResult.contentProviderError = true

We should probably also extract the notification related methods from the SyncManager to a separate class which can be used by Syncer and SyncManager or maybe even only the BaseSyncWorker