TouchInputMapper: s/mExternalStylusId/mFusedStylusPointerId

Use std::optional to hold the fused external stylus pointer id.

Bug: 246394583
Test: atest inputflinger_tests
Change-Id: Iebd9027639cb994a5e2916334e1c458aa45b9019
diff --git a/services/inputflinger/reader/mapper/TouchInputMapper.h b/services/inputflinger/reader/mapper/TouchInputMapper.h
index efbe885..d2faf3f 100644
--- a/services/inputflinger/reader/mapper/TouchInputMapper.h
+++ b/services/inputflinger/reader/mapper/TouchInputMapper.h
@@ -362,7 +362,10 @@
 
     // State provided by an external stylus
     StylusState mExternalStylusState;
-    int64_t mExternalStylusId;
+    // If an external stylus is capable of reporting pointer-specific data like pressure, we will
+    // attempt to fuse the pointer data reported by the stylus to the first touch pointer. This is
+    // the id of the pointer to which the external stylus data is fused.
+    std::optional<uint32_t> mFusedStylusPointerId;
     nsecs_t mExternalStylusFusionTimeout;
     bool mExternalStylusDataPending;
     // A subset of the buttons in mCurrentRawState that came from an external stylus.