Fix the symlink detection when signing TF.zip.

We used to check for 'attr >> 16 == 0xa1ff' (i.e. 0o120777) to detect
symlinks in the input target_files zip (TF.zip). This becomes broken
after we switch to soong_zip, which packs symlinks with 0o120700.

This CL fixes the issue by using stat.S_ISLNK() instead.

Note that we don't need to stage the files with the exact permission
bits as in the input TF.zip. Because this part is covered by mkbootfs
by using the canned or the compiled-in fs_config - as long as the
files/directories are accessible and the symlinks are created.

Bug: 38455129
Test: sign_target_files_apks.py on bullhead TF.zip. Check the
      checksums in SYSTEM/bin/install-recovery.sh.
Change-Id: I51c1fc9a257fb3f18c16c2ed71528abaa6f7d9c9