Add account sync summary.

- Check for the number of sync adapter that is enabled for the account and
update the summary text for the account sync preference accordingly.
- Add divider above the Remove Account button.

Merged-In: Ic333f62cce9aed0a72771324976ebe3d8e586287
Change-Id: I003d6dd0a070bf123f178b8d9ba7b6657466b905
Fix: 62862167
Test: make RunSettingsRoboTests