InputMapper refactor: ExternalStylusInputMapper

Add a factory method for ExternalStylusInputMapper to be configured on
initilisation

Test: m checkinput && atest libinput_tests inputflinger_tests
Bug: 256009910
Change-Id: If75377416f4f6d3954480bf4e32886f3daf7f1f3
(cherry picked from commit c55f75fc90e99c55a7133f82c407520f22937d96)
diff --git a/services/inputflinger/reader/InputDevice.cpp b/services/inputflinger/reader/InputDevice.cpp
index ec8a443..721bb0a 100644
--- a/services/inputflinger/reader/InputDevice.cpp
+++ b/services/inputflinger/reader/InputDevice.cpp
@@ -520,7 +520,7 @@
 
     // External stylus-like devices.
     if (classes.test(InputDeviceClass::EXTERNAL_STYLUS)) {
-        mappers.push_back(std::make_unique<ExternalStylusInputMapper>(contextPtr, readerConfig));
+        mappers.push_back(createInputMapper<ExternalStylusInputMapper>(contextPtr, readerConfig));
     }
     return mappers;
 }