Implements a full screen activity to add custom assets or NFTs.
Fix some minor issues to make it consistant with iOS and Desktop implementation:
* For Solana tokens the label `Token address` becomes `Mint address`.
* The new UI got rid of the cancel button as the same action can be achieved by tapping on the back arrow icon or using the system navigation controls.
* Token ID for ERC-721 is not shown for Solana NFTs.
* Text watcher has been improved moving its implementation into a class interface.
* Minor improvements to `NetworkSpinnerAdapter` class.
Simone Arpe
320adef986