Handle symlinks in sbox

Previously, sbox would follow the symlink and copy the symlink's target
into the sbox, instead of the symlink itself.

camera-its is a genrule that zips up a folder that includes a symlink,
so we want the symlink to still be there instead of a copy of the file
it points to.

Bug: 307824623
Test: ./build/soong/tests/genrule_sandbox_test.py camera-its
Change-Id: Ib81f6fd6745ec2c58580082c73e33760d5051f5e