Use a cross icon instead of an arrow for modal screens

[vfaramond]

Describe the bug The Send, Receive and Camera screens use the modal mode of react-navigation. The use of a cross icon in the top right corner of the screen is more suitable than using a back arrow on the top left corner of the screen.

To Reproduce Steps to reproduce the behavior:

1. Create a new wallet
2. Click on 'Send' or 'Receive'
3. See back arrow on the top left corner of the screen

Expected behavior The screen should display a cross icon on the top right of the screen.

Screenshots