[ActivityManager] Invalidate last screenshot when activity relaunched

Symptom:
The task thumbnail is not updated when activity relaunched.

Reproduce Steps:
1. Put device in portrait
2. Launch Calculator
3. Launch Recent App (the Calculator's screenshot is correct)
4. Rotate device to landscape
5. Click Calculator in Recent App to return to Calculator (Calculator has relaunched to landsacpe ui)
6. Launch Recent App again (the Calculator's screenshot is not updated)

Change-Id: I92e951ea2ee215c52ca6e50cf6f9e02deb787bce

services/core/java/com/android/server/am/ActivityStack.java

@@ -1819,6 +1819,9 @@ final class ActivityStack {
                     // Do over!
                     mStackSupervisor.scheduleResumeTopActivities();
                 }
+                if (next == mLastScreenshotActivity) {
+                    invalidateLastScreenshot();
+                }
                 if (mStackSupervisor.reportResumedActivityLocked(next)) {
                     mNoAnimActivities.clear();
                     if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();