Authentication added making it a completely functional project

[usman18]

1. Firebase phone authentication added. The OTP is directly detected from mobile and displayed in the edittexts(without user having to enter it) and sent for verification. If the OTP is sent to a number which is on some other device, then the user can enter the code manually.
2. Migrated entire project to Android X as all support libraries below or equal to version 28.0.0 will stop receiving further updates.
3. Structured the code, used proper camel case naming convention.
4. Documented functions wherever needed.

Developer Guide :
Completely plug and play.
All the developer has to do is place his/her own play-services.json file downloaded from firebase console and place it in the "app" directory.