android: Avoid the extra background on the icon, and set a different color for F-droid builds.

src/android/res/drawable/icon_fdroid.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+    <item
+        android:gravity="center"
+        android:width="256dp"
+        android:height="256dp">
+        <shape>
+            <solid
+                android:color="#0fa"/>
+        </shape>
+    </item>
+    <item android:drawable="@drawable/logo"
+        android:width="65dp"
+        android:height="65dp"
+        android:gravity="center"/>
+</layer-list>