android: Use Java 8 features

src/frontends/android/app/build.gradle

@@ -40,6 +40,10 @@ android {
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
         }
     }
+    compileOptions {
+        targetCompatibility 1.8
+        sourceCompatibility 1.8
+    }
 }
 
 dependencies {