releasetools: Resolve symlinks in IsEntryOtaPackage()

Test: Sign target files package where
      SYSTEM/product/media/bootanimation-dark.zip is a symlink to
      bootanimation.zip.
Change-Id: I4648c3c39c094cb090cbe337c566c3e9ad894691

tools/releasetools/sign_target_files_apks.py

@@ -271,6 +271,10 @@ def IsOtaPackage(fp):
 
 def IsEntryOtaPackage(input_zip, filename):
   with input_zip.open(filename, "r") as fp:
+    external_attr = input_zip.getinfo(filename).external_attr
+    if stat.S_ISLNK(external_attr >> 16):
+      return IsEntryOtaPackage(input_zip,
+          os.path.join(os.path.dirname(filename), fp.read().decode()))
     return IsOtaPackage(fp)