Use a sysprop to enable `TouchpadInputMapper`

The new touchpad behaviour can now be toggled using the `setprop`
command followed by a userspace reboot instead of having to change the
const and rebuild.

Bug: 251196347
Test: set the flag with setprop, check the gestures library logs appear
Test: atest inputflinger_tests
Change-Id: Ie4db00219e915fd059647d7541ef84e27e75bd86
diff --git a/services/inputflinger/reader/InputDevice.cpp b/services/inputflinger/reader/InputDevice.cpp
index 150a8aa..8d5377f 100644
--- a/services/inputflinger/reader/InputDevice.cpp
+++ b/services/inputflinger/reader/InputDevice.cpp
@@ -20,6 +20,9 @@
 
 #include <algorithm>
 
+#if defined(__ANDROID__)
+#include <android/sysprop/InputProperties.sysprop.h>
+#endif
 #include <ftl/flags.h>
 
 #include "CursorInputMapper.h"
@@ -209,9 +212,13 @@
     }
 
     // Touchscreens and touchpad devices.
-    // TODO(b/251196347): replace this with a proper flag.
-    constexpr bool ENABLE_NEW_TOUCHPAD_STACK = false;
-    if (ENABLE_NEW_TOUCHPAD_STACK && classes.test(InputDeviceClass::TOUCHPAD) &&
+    static const bool ENABLE_TOUCHPAD_GESTURES_LIBRARY =
+#if defined(__ANDROID__)
+            sysprop::InputProperties::enable_touchpad_gestures_library().value_or(false);
+#else
+            false;
+#endif
+    if (ENABLE_TOUCHPAD_GESTURES_LIBRARY && classes.test(InputDeviceClass::TOUCHPAD) &&
         classes.test(InputDeviceClass::TOUCH_MT)) {
         mappers.push_back(std::make_unique<TouchpadInputMapper>(*contextPtr));
     } else if (classes.test(InputDeviceClass::TOUCH_MT)) {