Preserves current search term in list result view even if user clears search box.

* Retains search term for current result set even if search text view is cleared or new text has been entered (but not submitted).
* Passes search term for current results from `PagerResultsFragment` to `ListResultsActivity` as an intent extra.
* Passes search term for current results from `ListResultsActivity` to `ListResultsFragment` as new instance parameter.
* Adds missing test for passing feature list from `ListResultsActivity` to `ListResultsFragment` as new instance parameter.
* Moves `ListResultsFragment` into `com.mapzen.search` package.