Fix NullPointer when factory debug flag enabled

When debug.screenshot_rounded_corners prop is true, some class fields
are not initialized through start(). And it causes a
NullPointerException during dumping ScreenDecorations.

Bug: 234549460
Test: 1. Enable debug.screenshot_rounded_corners
      2. Restart sysui process
      3. adb shell dumpsys activity service com.android.systemui
Change-Id: If0a31b19fb160de5bb717c1e76af2503d22c10af