Split CalendarRepository into Repository & DataSources scheme

CalendarRepository is currently a monolithic file that
 contains all operations related to Calendars.
 While it is true that the Repository is meant to be a source of truth.
 It is not the case to have all operations in it.

Instead, splitting the repositorys data accessor functions into
 datasources lowers the code complexity of the repository itself,
 letting it focus on proper tasks such as actually handling dataflow
 and truth.

A final adjustment is adding an interface between the actual
 implementation of the repository and the UI layer.
 This allows UI developers to not be overwhelmed looking at
 implementation code, and instead focus on accessing methods they need.

Another small change made in this is replacing the usage of LiveData
 in the data layer.
 LiveData is a UI level structure meant best to be used with old
 android views, not for the data layer.

Replacement with Kotlin Flow fits the proper scheme for
 what the data layer is tasked with.
 To lower the amount of changes, the UI layer handles the change of the
 Flow into LiveData via the `asLiveData()` extension function.
 This change also removes ContentProviderLiveData.kt, which is simply
 replaced with `callbackFlow`.

Copyright has also been updated for respective files.

Change-Id: Ib6344e43b9e7501d4551419cad870734e4e352bd