Add support for Android adaptive icons.

platform/android/java/app/AndroidManifest.xml

@@ -27,7 +27,7 @@
     <!-- Any tag in this line after android:icon will be erased when doing custom builds. -->
     <!-- If you want to add tags manually, do before it. -->
     <!-- WARNING: This should stay on a single line until the parsing code is improved. See GH-32414. -->
-    <application android:label="@string/godot_project_name_string" android:allowBackup="false" tools:ignore="GoogleAppIndexingWarning" android:icon="@drawable/icon" >
+    <application android:label="@string/godot_project_name_string" android:allowBackup="false" tools:ignore="GoogleAppIndexingWarning" android:icon="@mipmap/icon" >
 
         <!-- The following metadata values are replaced when Godot exports, modifying them here has no effect. -->
         <!-- Do these changes in the export preset. Adding new ones is fine. -->

platform/android/java/lib/AndroidManifest.xml

@@ -11,7 +11,7 @@
     </application>
 
     <instrumentation
-        android:icon="@drawable/icon"
+        android:icon="@mipmap/icon"
         android:label="@string/godot_project_name_string"
         android:name=".GodotInstrumentation"
         android:targetPackage="org.godotengine.godot" />

platform/android/java/lib/res/mipmap-anydpi-v26/icon.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+    <background android:drawable="@mipmap/icon_background"/>
+    <foreground android:drawable="@mipmap/icon_foreground"/>
+</adaptive-icon>