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
diff --git a/services/inputflinger/reader/mapper/ExternalStylusInputMapper.h b/services/inputflinger/reader/mapper/ExternalStylusInputMapper.h
index 841c437..97df02b 100644
--- a/services/inputflinger/reader/mapper/ExternalStylusInputMapper.h
+++ b/services/inputflinger/reader/mapper/ExternalStylusInputMapper.h
@@ -26,8 +26,10 @@
 
 class ExternalStylusInputMapper : public InputMapper {
 public:
-    explicit ExternalStylusInputMapper(InputDeviceContext& deviceContext,
-                                       const InputReaderConfiguration& readerConfig);
+    template <class T, class... Args>
+    friend std::unique_ptr<T> createInputMapper(InputDeviceContext& deviceContext,
+                                                const InputReaderConfiguration& readerConfig,
+                                                Args... args);
     virtual ~ExternalStylusInputMapper() = default;
 
     uint32_t getSources() const override;
@@ -46,6 +48,8 @@
 
     StylusState mStylusState;
 
+    explicit ExternalStylusInputMapper(InputDeviceContext& deviceContext,
+                                       const InputReaderConfiguration& readerConfig);
     [[nodiscard]] std::list<NotifyArgs> sync(nsecs_t when);
 };