Fix crash when taking screenshots from overview

When taking screenshots from overview, the current screenshot
wasn't being cleared, leading to a crash when we tried to add the
view to the window again. Clearing the screenshot first (as we do
when taking screenshots from hardware/global actions) fixes the
problem.

Fixes: 151153057
Test: manual (verified that error occurs if you tap the screenshot
button while the UI is up pre-fix, and crash does not happen
post-fix)

Change-Id: Ia33344ce19c912241e18d3053ccdc51845603ca3

packages/SystemUI/src/com/android/systemui/screenshot/GlobalScreenshot.java

@@ -347,6 +347,7 @@ public class GlobalScreenshot implements ViewTreeObserver.OnComputeInternalInset
     void handleImageAsScreenshot(Bitmap screenshot, Rect screenshotScreenBounds,
             Insets visibleInsets, int taskId, Consumer<Uri> finisher) {
         // TODO use taskId and visibleInsets
+        clearScreenshot("new screenshot requested");
         takeScreenshot(screenshot, finisher, screenshotScreenBounds);
     }