TODO-MVP: getTaskWithId could return dirty task

Where is check `mCacheIsDirty` or why is it not necessary?
```
@Nullable
private Task getTaskWithId(@NonNull String id) {
checkNotNull(id);
if (mCachedTasks == null || mCachedTasks.isEmpty()) {
return null;
} else {
return mCachedTasks.get(id);
}
}
```

Start by tracing the private getTaskWithId(String id) method and the mCachedTasks and mCacheIsDirty state it references. Determine whether a dirty cache can return an outdated Task, then identify the existing test or call path that demonstrates the expected behavior. Done means the cache-dirty behavior is resolved or explicitly shown to be unnecessary.

Written by the indexing model from the issue text.