To integrate UPI SDK in you android app you have to partner with a bank which is offering this service. Only UPI enabled banks are allowed to act as PSP (Payment service Provider). Some banks which are offering this service are: Yes Bank, RBL, ICICI Bank, Axis Bank.
Once you have the SDK it's pretty much Plug n Play and will hardly require a day's work. Although the bank that is offering you the SDK will review the developed app to see if it matches their security standards.
