Fix get_abs_build_var to work from below $ANDROID_BUILD_TOP

I tried running gdbclient from a subdirectory of $ANDROID_BUILD_TOP
and found that the absolute paths generated by get_abs_build_var
included my subdirectory's path. This fixed get_abs_build_var to
changing directory in a subshell to $ANDROID_BUILD_TOP before invoking
make.

Change-Id: I12b9d9135ac7c044c21c88ed5705b22b0700c039