added support for java8 features

lib/android/app/build.gradle

     compileSdkVersion 25
     buildToolsVersion "27.0.1"
 
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
     defaultConfig {
         minSdkVersion 16
         targetSdkVersion 25

lib/android/app/src/main/java/com/reactnativenavigation/NavigationActivity.java

 package com.reactnativenavigation;
 
-import android.os.Bundle;
-import android.support.annotation.Nullable;
-import android.support.v7.app.AppCompatActivity;
-import android.view.KeyEvent;
-import android.widget.Toast;
+import android.os.*;
+import android.support.annotation.*;
+import android.support.v7.app.*;
+import android.view.*;
+import android.widget.*;
 
-import com.facebook.react.modules.core.DefaultHardwareBackBtnHandler;
-import com.reactnativenavigation.viewcontrollers.Navigator;
+import com.facebook.react.modules.core.*;
+import com.reactnativenavigation.viewcontrollers.*;
 
 public class NavigationActivity extends AppCompatActivity implements DefaultHardwareBackBtnHandler {
 	private Navigator navigator;
 
 	@Override
 	public void invokeDefaultOnBackPressed() {
-        if (!navigator.handleBack()) {
-		    super.onBackPressed();
-        }
+		if (!navigator.handleBack()) {
+			super.onBackPressed();
+		}
 	}
 
 	@Override
 	public void onBackPressed() {
-        app().getReactGateway().onBackPressed();
+		app().getReactGateway().onBackPressed();
 	}
 
 	@Override

lib/android/app/src/test/java/com/reactnativenavigation/TestApplication.java

 package com.reactnativenavigation;
 
-import android.app.Application;
+import android.app.*;
 
 public class TestApplication extends Application {
 	@Override

playground/android/app/build.gradle

     compileSdkVersion 25
     buildToolsVersion "27.0.1"
 
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
     defaultConfig {
         applicationId "com.reactnativenavigation.playground"
         minSdkVersion 16