Android: allow Android app building to use java 8 features

Enable Gradle builds to use Java 8 features like lambda expression by
default.

Task-number: QTBUG-86282
Pick-to: 5.15
Change-Id: Ib497b8a789690fcca6e13ea4995e5966331c6c0d
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/android/templates/build.gradle

@@ -55,6 +55,11 @@ android {
         options.incremental = true
     }
 
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
     lintOptions {
         abortOnError false
     }