java: fix bindings generator

- fix imports override.
  Problem is observed with BoostDesc.

- add Ptr<> handling (constructor is protected from other packages).
  Observed in ximgproc:
      Ptr<StereoMatcher> createRightMatcher(Ptr<StereoMatcher> matcher_left)"
  where, "StereoMather" is from another package (calib3d)

modules/java/generator/templates/java_class_inherited.prolog

@@ -10,3 +10,6 @@ $annotation
 public class $jname extends $base {
 
     protected $jname(long addr) { super(addr); }
+
+    // internal usage only
+    public static $jname __fromPtr__(long addr) { return new $jname(addr); }