Account refactoring follow-up

- Check if there's really a new or removed account in
updateAccountsInBackground() before doing everything else.
If there's none, we'll skip re-scanning directories too.

- Preheat the account cache when the provider starts.

- Use account_id for the account filter query parameters.  The contacts URI
now supports the parameters too.

Change-Id: I12e1116890df4c20b354618acfaee9dc009dc68e
6 files changed