Fix the double query issue on phone

The issue was that fragments had separate methods for search: setSearchMode()
and setQueryString().  But now that fragments will never be in search mode
when query is empty, there's no need to expose both methods.

Changed setSearchMode() to non-public, and let setQueryString() call it
when necessary, so now we don't have to issue unnecessary queries.

Note this CL doesn't modify adapters, so they still have setSearchMode()
and setQueryString() as separate, public methods.

Also did a bit of cleaning up on how PeopleActivity handles search mode.
(When I worked on it first time I wasn't too familiar with the code, so was
afraid to change existing code too much, which left the code unnecessarily
complicated.)

Also removed all fragment.setSearchMode()/setQueryString() calls in
ContactSelectionActivity.configureListFragment().  As far as I checked
there's no way to invoke this activity with reqeust.isSearchMode/getQueryText
set.

Also removed ContactEntryListFragment.get/setContactRequest, which weren't used
anywhere.

Bug 5274171

Change-Id: I794db7ed54cb6b8f45d69430ec2f77e7fa83fb8c