Fix lunch choices w/zsh

zsh doesn't split variables (by default), but it will split command
output. bash will split both. Apparently a portable way to do this in
both is to just turn it into another command: "$(echo $var)".

Test: source build/envsetup.sh; lunch (see list; repeat in bash and zsh)
Change-Id: I9716a8d77aefdf05da8923deb9e37e0b6132b549

envsetup.sh

@@ -580,7 +580,7 @@ function print_lunch_menu()
 
     local i=1
     local choice
-    for choice in $choices
+    for choice in $(echo $choices)
     do
         echo "     $i. $choice"
         i=$(($i+1))