This happens when you open the Windows > Prefs > Android panel
while an SDK is initially loading or when you change the
SDK in the pref panel. The target change listener was not
properly removed since the field was not properly disposed.
This also removed the multiple selection handling in the
SdkTargetSelector, which we never use. In the unlikely event
we want to use it later, it would be trivial to add it back.
Original author: raphael
Merged from: //branches/cupcake/...
Automated import of CL 146634
