Show spinner only when user pulls down (1/2)

1. show spinner only when user pulls down;
2. stop showing spinner if sync is not finished after 30s;
3. add max time to show spinner into Flags.

Bug: 28625097
Bug: 31993722

Test: Manually verified scenarios:
Only pulling down can trigger spinner to show up, spinner won't be
shown for any other background sync(add/remove account, edit contacts);
refresh an account with over 2000 contacts, spinner disappears after
30s;
refresh without internet connection, spinner disappears after 30s;
refresh twice within 30s, spinner disappears after 30s since the second
refreshing.

Change-Id: Ib88c14a40fc8787527baa9d5ac99da910fdd352b