Update Android custom template build configuration.

platform/android/java/app/build.gradle

@@ -56,6 +56,11 @@ android {
     compileSdkVersion versions.compileSdk
     buildToolsVersion versions.buildTools
 
+    compileOptions {
+        sourceCompatibility 1.8
+        targetCompatibility 1.8
+    }
+
     defaultConfig {
         // Feel free to modify the application id to your own.
         applicationId getExportPackageName()

platform/android/java/app/settings.gradle

@@ -0,0 +1,2 @@
+// Empty settings.gradle file to denote this directory as being the root project
+// of the Godot custom build.