Performance Improvement | Documentation Update

[jeremiahseun]

I made few changes that makes development relatively easy. The following:

• Change Transaction Charge Type value (From String to enum)
• Change the Payment Option to a List (We won't have to guess what type of payment option are available. Now, it is a list that you add whatever option you want to add.
• Upgrade the Example Compile SDK Version (From 32 to 33, so as the example app will run fine in Android SDK 33).
• Few changes and correction - Optimization purposes.