Datalayer for Desktop windowing education

As the data we need is required to persist through device reboots, we can not rely on in-memory storage. Hence we are using Android ProtoDataStore.
WindowingEducationDatastoreRepository- Repository that accesses datastore by reading values and writing into it.
More details can be read at go/dw-education-design-doc.

Bug: 352552060, 352552062
Test: AppHandleEducationDatastoreRepositoryTest
Flag: com.android.window.flags.enable_desktop_windowing_app_handle_education
Change-Id: I4cd7f7a45b9ec89b56907d65acd2e6253081520f