Fix zsh compatibility issue in build/envsetup.sh

Arrays are zero indexed in bash and one indexed in zsh by default. This
leads to an off-by-one issue in the `godir` and `choosevariant` commands: when
prompted to chose an option, in order to pick option "n" you have to input
"n+1".

In those two specific instances, by using "substring expansion" instead of
array indexing, one can get consistent behaviour between bash and zsh
(equivalent to zero indexing).

Test: manual -
 # godir
 $ zsh
 $ source build/envsetup.sh
 $ godir SurfaceFlinger # There should be many options
 # Pick 1, check that you end up in the right location
 $ godir SurfaceFlinger.cpp # There should be only one possibility
 # make sure you end up in the right location
 # repeat with bash (to ensure compatibility is not broken)

 # choosevariant
 $ zsh
 $ source build/envsetup.h
 $ choosevariant # pick whatever variant you want
 $ printconfig # make sure the variant matches your choice
 # repeat with bash (to ensure compatibility is not broken)

Change-Id: I998d8fb48b708066b6db28a2129a2b09785fb0b1