Check how Android launchers that prefer square icons handle ours

[gnprice]

This issue is derived from #3075 and #2993, specifically my last comment on the latter:

Empirically, on my device on stock Android 9, our icon with [a targetSdkVersion of 26] shows up perfectly fine in the launcher the same as it always has, but maybe some launchers that prefer square icons will behave less well?
[...]
At some point we should check how a recent version (19.2.102, released today to Play Store prod, has the targetSdkVersion upgrade) looks on some other launchers, and might find some adjustments are needed and circle back to this area.

If any adjustments are needed, they'd likely involve the android:roundIcon attribute in the manifest. E.g., the RN upstream template gained this line between v0.57 and v0.59 (making this a sort of followup task to #3399):

     <application
...
+      android:roundIcon="@mipmap/ic_launcher_round"

We haven't prioritized this because the point of that change in Android is that the stock launcher has started preferring round icons -- and our icon is already round, so it works great with no adjustments.

A related very low priority, also taken from #2993:

Also the debug icon gets adjusted and looks less good, but that's a very low priority.