Fix potential crash when exit splash screen

- Make sure mDecorView is attached before calling its
  WindowInsetsController.
- Amend modifier declare sequence for a better code style.

Bug: 258270145
Bug: 256766422
Test: atest SplashscreenTests StartingSurfaceDrawerTests
Change-Id: I4d3b184252ed9516827812adfa0c76e07a6527e7
Merged-In: I4d3b184252ed9516827812adfa0c76e07a6527e7

libs/WindowManager/Shell/src/com/android/wm/shell/startingsurface/StartingSurfaceDrawer.java

@@ -430,7 +430,8 @@ public class StartingSurfaceDrawer {
         }
 
         @Override
-        public @Nullable SplashScreenView get() {
+        @Nullable
+        public SplashScreenView get() {
             synchronized (this) {
                 while (!mIsViewSet) {
                     try {
@@ -691,7 +692,7 @@ public class StartingSurfaceDrawer {
         private final TaskSnapshotWindow mTaskSnapshotWindow;
         private SplashScreenView mContentView;
         private boolean mSetSplashScreen;
-        private @StartingWindowType int mSuggestType;
+        @StartingWindowType private int mSuggestType;
         private int mBGColor;
         private final long mCreateTime;
         private int mSystemBarAppearance;
@@ -732,7 +733,7 @@ public class StartingSurfaceDrawer {
 
         // Reset the system bar color which set by splash screen, make it align to the app.
         private void clearSystemBarColor() {
-            if (mDecorView == null) {
+            if (mDecorView == null || !mDecorView.isAttachedToWindow()) {
                 return;
             }
             if (mDecorView.getLayoutParams() instanceof WindowManager.LayoutParams) {