[Code health] Idea: Simplify local db interfaces and impls

The current structure of `ground/src/main/java/com/google/android/ground/persistence/local` is:

```
├── LocalDataConsistencyException.kt
├── LocalDataStore.kt
├── LocalDataStoreModule.kt
├── LocalDatabaseModule.kt
├── LocalValueStore.kt
├── room
│   ├── IntEnum.java
│   ├── LocalDataStoreException.kt
│   ├── LocalDatabase.kt
│   ├── RoomLocalDataStore.kt
│   ├── converter
│   │   ├── ConverterExt.kt
│   │   ├── JsonArrayTypeConverter.kt
│   │   ├── JsonObjectTypeConverter.kt
│   │   ├── ResponseDeltasConverter.kt
│   │   ├── ResponseJsonConverter.kt
│   │   ├── ResponseMapConverter.kt
│   │   └── StyleTypeConverter.kt
│   ├── dao
│   │   ├── BaseDao.kt
│   │   ├── BaseMapDao.kt
│   │   ├── JobDao.kt
│   │   ├── LocationOfInterestDao.kt
│   │   ├── LocationOfInterestMutationDao.kt
│   │   ├── MultipleChoiceDao.kt
│   │   ├── OfflineAreaDao.kt
│   │   ├── OptionDao.kt
│   │   ├── SubmissionDao.kt
│   │   ├── SubmissionMutationDao.kt
│   │   ├── SurveyDao.kt
│   │   ├── TaskDao.kt
│   │   ├── TileSetDao.kt
│   │   └── UserDao.kt
│   ├── entity
│   │   ├── AuditInfoEntity.kt
│   │   ├── BaseMapEntity.kt
│   │   ├── GeometryEntity.kt
│   │   ├── JobEntity.kt
│   │   ├── LocationOfInterestEntity.kt
│   │   ├── LocationOfInterestMutationEntity.kt
│   │   ├── MultipleChoiceEntity.kt
│   │   ├── OfflineAreaEntity.kt
│   │   ├── OptionEntity.kt
│   │   ├── SubmissionEntity.kt
│   │   ├── SubmissionMutationEntity.kt
│   │   ├── SurveyEntity.kt
│   │   ├── TaskEntity.kt
│   │   ├── TileSetEntity.kt
│   │   └── UserEntity.kt
│   ├── models
│   │   ├── Coordinates.java
│   │   ├── EntityState.java
│   │   ├── GeometryType.kt
│   │   ├── MultipleChoiceEntityType.java
│   │   ├── MutationEntitySyncStatus.java
│   │   ├── MutationEntityType.java
│   │   ├── OfflineAreaEntityState.java
│   │   ├── TaskEntityType.java
│   │   ├── TileSetEntityState.java
│   │   └── UserDetails.java
│   ├── relations
│   │   ├── JobEntityAndRelations.kt
│   │   ├── SurveyEntityAndRelations.kt
│   │   └── TaskEntityAndRelations.kt
│   └── stores
│   ├── RoomLocalLocationOfInterestMutationStore.kt
│   ├── RoomLocalOfflineAreaStore.kt
│   ├── RoomLocalSubmissionMutationStore.kt
│   ├── RoomLocalSurveyStore.kt
│   ├── RoomLocalTileSetStore.kt
│   └── RoomLocalUserStore.kt
└── stores
├── LocalLocationOfInterestMutationStore.kt
├── LocalMutationStore.kt
├── LocalOfflineAreaStore.kt
├── LocalStore.kt
├── LocalSubmissionMutationStore.kt
├── LocalSurveyStore.kt
├── LocalTileSetStore.kt
└── LocalUserStore.kt
```

A few questions that might help understand how to simplify this structure:
- [ ] If Stores are 1:1 with Daos, what value-add do they provide?
- [ ] At least one operation in `LocalDataStore` combines data from multiple model-based stores. If that's the case, should `LocalDataStore` be scoped to coordination layer rather than domain-model (i.e., implementation of local persistence vs. implementation of local persistence of a specific item)?
- [ ] Does the addition of multiple interfaces and related implementations for each domain object increase or decrease readability and complexity? Is it obvious when to look in `LocalDataStore`, `RoomLocalDataStore`, `LocalStore`, `LocalMutationStore`, `RoomLocalSurveyStore`, or `LocalSurveyStore`, for example?
- [ ] What are "models" and how do they differ from "entities"? If they're just subcomponents of entities, should they live alongside them in the same package?

@scolsen and I were discussing most of these today, @JSunde @shobhitagarwal1612 curious to hear your thoughts.

Note that I'm also having trouble adding coroutines gradually since use of Rx is prescribed by the shared interface.