vndk-def: Fix is_zipfile() for Python 3.5

Python 3.5 adopts a fancy algorithm to detect the zip file.  It will
search for the magic words in the file, i.e. magic word does not have to
be at the beginning.  This commit adds an extra check to make sure "PK",
the magic word for zip file, is at the beginning of the file.

Test: ./vndk_definition_tool.py apk-deps --system ... --vendor ...
Change-Id: I3e633709bc39187ae807aef97c6e0396d02266ea

vndk/tools/definition-tool/vndk_definition_tool.py

@@ -882,8 +882,21 @@ class DexFileReader(object):
 
 
     @classmethod
-    def enumerate_dex_strings_apk(cls, apk_file):
-        with zipfile.ZipFile(apk_file, 'r') as zip_file:
+    def _read_first_bytes(cls, apk_file, num_bytes):
+        try:
+            with open(apk_file, 'rb') as fp:
+                return fp.read(num_bytes)
+        except IOError:
+            return b''
+
+    @classmethod
+    def is_zipfile(cls, apk_file_path):
+        magic = cls._read_first_bytes(apk_file_path, 2)
+        return magic == b'PK' and zipfile.is_zipfile(apk_file_path)
+
+    @classmethod
+    def enumerate_dex_strings_apk(cls, apk_file_path):
+        with zipfile.ZipFile(apk_file_path, 'r') as zip_file:
             for name in cls.generate_classes_dex_names():
                 try:
                     with zip_file.open(name) as dex_file:
@@ -3186,7 +3199,7 @@ class ApkDepsCommand(ELFGraphCommand):
 
     def scan_apk_deps(self, libnames, system_dirs, vendor_dirs):
         for ap, path in self._enumerate_paths(system_dirs, vendor_dirs):
-            if not zipfile.is_zipfile(path):
+            if not DexFileReader.is_zipfile(path):
                 continue
             libs = set()
             strs = set(DexFileReader.enumerate_dex_strings_apk(path))