Kiwix-android: Move to single activity architecture

[Frans-Lukas]

Is your feature request related to a problem? Please describe.

The single activity architecture is officially recommended by google and brings many benefits, such as faster UI transitions, easier shared data and a simplified lifecycle.

Describe the solution you'd like

All activities except for KiwixMainActivity and CustomMainActivity should be converted to fragments and use the navigation component for navigation.

[kelson42]

@Frans-Lukas @macgills Do we have created all sub-tickets? Does that make sense to keep that one open?

[macgills]

I think there is still 1 activity without a ticket, CustomDownload

[kelson42]

@Frans-Lukas @macgills Is that something we have any chance to complete for 3.4 and needs to be done within 3.4?

[macgills]

It is not necessary but would make things more cohesive, we are nearly there but understandably it is moving slower now that GSOC is complete.

[kelson42]

@macgills Thank you for the feedback. It seems that we will have anyway to postpone the prod rollout of 3.4 anyway of a few weeks. I'm really glad and thankfull that @Frans-Lukas runs this effort to the end. Please consider a plan B if you believe this is going to take more than 2 or 3 additional weeks.

[macgills]

I don't forsee this taking 2-3 weeks, the tickets to consider this done are in Review currently and then we can merge the one-activity branch

[Frans-Lukas]

Agreed!

[kelson42]

BRAVO!!!!