iMessage is Apple’s proprietary internet messaging service. iMessages can only be sent and received between Apple devices.
You cannot share messages between an Apple device and an Android device since Apple’s iOS continuity feature is not available for Android (it is an Apple iOS and Mac OS feature only), nor can an Android device use iCloud for messages in iCloud.
So an iPad cannot be paired with an Android cell phone to send/receive SMS/MMS texts, like an iPad which is paired with an iPhone can do. An owner of an iPad and an Android phone will find the iPad is solely restricted to iMessage and other internet based messaging services (Skype, Google Hangouts, etc).