Add parsing metadata from OTA package

Since Oreo, there is no `system/build.prop`, it is packaged into
`system.new.dat`. So timestamp generation hasn't worked for years now.

The OTA metadata on the other hand stores this timestamp (called
`post-timestamp`), so let's parse that instead.

gen_mirror_json.py

@@ -26,11 +26,13 @@
     sha256 = hashlib.sha256()
     for buf in iter(lambda: data.read(128 * 1024), b''):
         sha256.update(buf)
+
         try:
             with zipfile.ZipFile(f'{FILE_BASE}/{filename}', 'r') as update_zip:
-                build_prop = update_zip.read('system/build.prop').decode('utf-8')
-                timestamp = int(re.findall('ro.build.date.utc=([0-9]+)', build_prop)[0])
-        except Exception:
+                build_prop = update_zip.read('META-INF/com/android/metadata').decode('utf-8')
+                timestamp = (re.findall('post-timestamp=([0-9]+)', build_prop)[0])
+        except Exception as e:
+            print(e)
             timestamp = int(mktime(datetime.strptime(builddate, '%Y%m%d').timetuple()))
 
     builds.setdefault(device, []).append({