refactor: 调整主窗口背景设置逻辑与布局透明度

- MainActivity: 将背景设置逻辑从 onCreate 移至 onPostCreate
- activity_main.xml: RecyclerView 背景颜色修改为透明

gallery/src/main/java/cc/winboll/studio/gallery/MainActivity.java

@@ -51,11 +51,6 @@ public class MainActivity extends AppCompatActivity {
         setContentView(R.layout.activity_main);
         LogUtils.d(TAG, "onCreate");
         
-        View content = findViewById(android.R.id.content);
-        if (content != null) {
-            content.setBackground(BackgroundUtils.getInstance().getDrawable());
-        }
-        
         Toolbar toolbar = findViewById(R.id.toolbar);
         setSupportActionBar(toolbar);
         
@@ -104,6 +99,15 @@ public class MainActivity extends AppCompatActivity {
         checkAndRequestPermissions();
     }
     
+    @Override
+    protected void onPostCreate(Bundle savedInstanceState) {
+        super.onPostCreate(savedInstanceState);
+        View content = findViewById(android.R.id.content);
+        if (content != null) {
+            content.setBackground(BackgroundUtils.getInstance().getDrawable());
+        }
+    }
+    
     private void checkAndRequestPermissions() {
         LogUtils.i(TAG, "checkAndRequestPermissions");
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {

gallery/src/main/res/layout/activity_main.xml

@@ -27,7 +27,7 @@
             android:id="@+id/recycler_view"
             android:layout_width="match_parent"
             android:layout_height="match_parent"
-            android:background="@color/black"/>
+            android:background="@android:color/transparent"/>
 
     </LinearLayout>