Prevent NPE on ExpandableNotificationRow.

An NPE could happen, related to falsing, if the very
first gesture to happen is a tap on an ExpandableNotificationRow.

Fixes: 187196123
Test: atest SystemUITests
Change-Id: I8a82fc4d85ba5676a6c198a0b6ddb67009c8ad8e

packages/SystemUI/src/com/android/systemui/classifier/FalsingDataProvider.java

@@ -49,7 +49,7 @@ public class FalsingDataProvider {
 
     private TimeLimitedMotionEventBuffer mRecentMotionEvents =
             new TimeLimitedMotionEventBuffer(MOTION_EVENT_AGE_MS);
-    private List<MotionEvent> mPriorMotionEvents;
+    private List<MotionEvent> mPriorMotionEvents = new ArrayList<>();
 
     private boolean mDirty = true;
 

packages/SystemUI/tests/src/com/android/systemui/classifier/FalsingDataProviderTest.java

@@ -290,4 +290,11 @@ public class FalsingDataProviderTest extends ClassifierTest {
         mDataProvider.onMotionEvent(appendDownEvent(0, 200));
         verify(listener).onGestureFinalized(100);
     }
+
+    @Test
+    public void test_GetPriorEventsEarly() {
+        // Ensure that if we ask for prior events before any events were added, we at least get
+        // an empty array.
+        assertThat(mDataProvider.getPriorMotionEvents()).isNotNull();
+    }
 }