Fixing no gradient player background when using "pure black" theme.

### Fixes #462 - `useBlackBackground` now considers `playerBackground` state - Player background updates dynamically based on: (`mediaMetadata`, `playerBackground`, `darkTheme`, `useBlackBackground`) to cover all possible theme configurations and state changes. - Introduces `hasGradientColours` to represent `gradientColors.size >= 2` for readability. - Refactors `onBackgroundColor` calculation into separate code segments for better readability ### Additional packages: None ### Screenshots: #### Light Mode <img src="https://github.com/user-attachments/assets/ca082d77-c33c-4e41-b387-c23e56f96e31" width="360" height="640"> #### Light Mode + Dynamic <img src="https://github.com/user-attachments/assets/a7506543-11db-4e83-8613-287a2909c690" height="640" width="360"> #### Light Mode + Gradient <img src="https://github.com/user-attachments/assets/39eadbda-5a4c-4d74-af90-b4f210173d82" height="640" width="360"> #### Light Mode + Dynamic + Gradient <img src="https://github.com/user-attachments/assets/aed2d0d2-e565-4e72-8d8b-3764ace9e9da" width="360" height="640"> --- #### Dark Mode <img src="https://github.com/user-attachments/assets/2a341d85-0bc5-42f4-b2be-3ead655bbdb0" height="640" width="360"> #### Dark Mode + Dynamic <img src="https://github.com/user-attachments/assets/58992cf6-157e-4085-bf1c-5e44f5400f66" height="640" width="360"> #### Dark Mode + Gradient (with/without Pure Black) <img src="https://github.com/user-attachments/assets/caa43a25-8e35-4976-890d-f28389afda8d" height="640" width="360"> #### Dark Mode + Dynamic + Gradient (with/without Pure Black) <img src="https://github.com/user-attachments/assets/644176b9-aac3-4b63-a0ad-4286d54c46fc" height="640" width="360"> #### Dark Mode + Pure Black <img src="https://github.com/user-attachments/assets/f7689375-4ae3-4b66-b453-7712730f4e80" height="640" width="360"> #### Dark Mode + Dynamic + Pure Black <img src="https://github.com/user-attachments/assets/2b38abf7-862c-4f88-916c-b381fd296ea0" height="640" width="360">