Exorcise notification panel ghosts.

Bug: 5105599

Don't clear slot data when slot is no longer in use.

The kernel will not resend absolute slot values if they are
unchanged, even if the slot was previously marked unused.

Because the protocol is stateful, this could cause problems if
evdev drops events (marked by SYN_DROPPED) since we cannot
query the initial values of the slots, but there's nothing
we can do.

Change-Id: Ie13e68097fb8ed2542c2f60338b499082cf9e77f

services/input/InputReader.cpp

@@ -1335,7 +1335,7 @@ void MultiTouchMotionAccumulator::configure(size_t slotCount, bool usingSlotsPro
 
 void MultiTouchMotionAccumulator::clearSlots(int32_t initialSlot) {
     for (size_t i = 0; i < mSlotCount; i++) {
-        mSlots[i].clearIfInUse();
+        mSlots[i].clear();
     }
     mCurrentSlot = initialSlot;
 }
@@ -1396,7 +1396,9 @@ void MultiTouchMotionAccumulator::process(const RawEvent* rawEvent) {
                 break;
             case ABS_MT_TRACKING_ID:
                 if (mUsingSlotsProtocol && rawEvent->value < 0) {
-                    slot->clearIfInUse();
+                    // The slot is no longer in use but it retains its previous contents,
+                    // which may be reused for subsequent touches.
+                    slot->mInUse = false;
                 } else {
                     slot->mInUse = true;
                     slot->mAbsMTTrackingId = rawEvent->value;
@@ -1430,12 +1432,6 @@ MultiTouchMotionAccumulator::Slot::Slot() {
     clear();
 }
 
-void MultiTouchMotionAccumulator::Slot::clearIfInUse() {
-    if (mInUse) {
-        clear();
-    }
-}
-
 void MultiTouchMotionAccumulator::Slot::clear() {
     mInUse = false;
     mHaveAbsMTTouchMinor = false;

services/input/InputReader.h

@@ -696,7 +696,6 @@ public:
         int32_t mAbsMTToolType;
 
         Slot();
-        void clearIfInUse();
         void clear();
     };