Fix wrong target launches in Chooser.

The service target row was sometimes overfull - with more targets in
the list than were displayed - when we used each row's offset
to calculate which target was selected to launch.
This fix makes sure we are not overfilling.

Test: ChooserActivityTest & manual repro -- use targets from
alphabetical section, make sure you have at least one old-API target
available
Fixes: 170518649, 165944833

Change-Id: I8a554197c01f4eadbeac43742984e4c86ca49601