Nexus 7 recognized as unknown Serial Number (Solved)

If your device cannot be recognized by ADT (or adb), ???????????? shown on Serial number.

In my case, solve it by setting USB computer connection to either Media device (MTP) or Camera (PTP). Don't disable both.

• Setting
• select Storage
• click on the menu icon on upper right corner, and click the USB computer connection
• Enable either Media device (MTP) or Camera (PTP)

Related: To enable developer options on KitKat (Android 4.4), refer to the post.