LocalDataStore: reusable method to acquire content provider

[rfc2822]

Currently, the content provider for a LocalDataStore is acquired by the code that uses it, namely Syncer.

However there will be multiple locations where acquiring the content provider is needed:

• Syncer
• UnifiedPushReceiver (in [Push] Support sync-token #1147)

So acquiring the content provider should be in a reusable class that is responsible to do so, possibly as LocalDataStore.acquireProvider(). This will probably also require to move the authority from the Syncer to the LocalDataStore.