Upgrade RxJava from version 2 to 3

[Akash-Ramjyothi]

Description:

• Currently, the android project is using an outdated RxJava version 2 instead of version 3.
• Making this library up to date is crucial to replace the otto event bus and for future features to be easily made and integrated.
• There are various benefits of RxJava 3 over 2 such as Threading, Error handling, Caching and Cancellation are made much easier.

Maybe fixed by: #933 .