Replace System_SendDebugOutput with a registered callback

I normally try to avoid registrations when not needed, but in this case
only headless uses this, so it's motivated.

android/jni/app-android.cpp

@@ -291,9 +291,6 @@ void System_LaunchUrl(LaunchUrlType urlType, std::string_view url) {
 	}
 }
 
-bool System_SendDebugOutput(std::string_view data) { return false; }
-void System_SendDebugScreenshot(const uint8_t *data, int width, int height) {}
-
 std::string System_GetProperty(SystemProperty prop) {
 	switch (prop) {
 	case SYSPROP_NAME: