Fix F-Droid not displaying logo (#2079)

# Description

It seems F-Droid doesn't display Syncthing's logo at their website and
in their client (at least on my end it is not shown).

According to the Wayback Machine, F-Droid did show the logo in the past,
but it stopped being displayed between October and December of 2022:


https://web.archive.org/web/20221027141012if_/https://f-droid.org/en/packages/com.nutomic.syncthingandroid/
versus

https://web.archive.org/web/20221208220952if_/https://f-droid.org/en/packages/com.nutomic.syncthingandroid/

This PR adds the Syncthing logo to the location expected by F-Droid, as
per the [Triple T
structure](https://f-droid.org/en/docs/All_About_Descriptions_Graphics_and_Screenshots/#triple-t-structure)
which you seem to be using.

# Changes

* Create 512x512 PNG from
[ic_launcher_full.svg](https://github.com/syncthing/syncthing-android/blob/main/graphics/ic_launcher_full.svg)
using Inkscape (and compress using zopfli)
* Create icon directory according to the [Triple T
structure](https://f-droid.org/en/docs/All_About_Descriptions_Graphics_and_Screenshots/#triple-t-structure)
* Copy PNG to appropriate location