Implement UI prototype for app sharing

Subtask of #1081 (closed) 1 acitivty + 3 fragments + 1 troubleshooting fragment

changed milestone to %Install via Bluetooth or Wi-Fi

added Sponsor 6 T: task labels

added 1 design

The design is discussed in #2011 (closed)

changed title from Implement UI prototype to Implement UI prototype for app sharing

@sebkur For reference, the Figma design specs are here: https://www.figma.com/file/NgBKOW1GcLk1NXzHQBcYdY/Offline-App-Installer-Sharing?node-id=1%3A5491

Colours are probably outdated though, so just use the default colours of the dark theme.

started a discussion on Image_Pasted_at_2021-5-3_19-04.png

added S: in progress label

assigned to @grote

added S: in code review label and removed S: in progress label

mentioned in merge request !1459 (merged)

closed

removed S: in code review label

added R: fixed label

changed due date to July 31, 2021