Allow touchpads to expose associated display

After landing ag/24209603, a touchpad test started failing; the
reason is that the device never exposed the associated display id,
even though it became associated to a viewport. This CL makes the
necessary changes so the associated display id is properly
propagated.

Bug: 293488187
Test: atest of the failing test
Change-Id: Ia1480012bd339de92abd284c6d00f7ac93505735
diff --git a/services/inputflinger/reader/mapper/TouchpadInputMapper.h b/services/inputflinger/reader/mapper/TouchpadInputMapper.h
index 73ca5af..47d712e 100644
--- a/services/inputflinger/reader/mapper/TouchpadInputMapper.h
+++ b/services/inputflinger/reader/mapper/TouchpadInputMapper.h
@@ -64,6 +64,8 @@
     using MetricsIdentifier = std::tuple<uint16_t /*busId*/, uint16_t /*vendorId*/,
                                          uint16_t /*productId*/, uint16_t /*version*/>;
 
+    std::optional<int32_t> getAssociatedDisplayId() override;
+
 private:
     void resetGestureInterpreter(nsecs_t when);
     explicit TouchpadInputMapper(InputDeviceContext& deviceContext,
@@ -102,6 +104,11 @@
     std::set<int32_t> mLastFrameTrackingIds;
     // Tracking IDs for touches that have at some point been reported as palms by the touchpad.
     std::set<int32_t> mPalmTrackingIds;
+
+    // The display that events generated by this mapper should target. This can be set to
+    // ADISPLAY_ID_NONE to target the focused display. If there is no display target (i.e.
+    // std::nullopt), all events will be ignored.
+    std::optional<int32_t> mDisplayId;
 };
 
 } // namespace android