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.prolog

@@ -13,3 +13,6 @@ public class $jname {
     protected $jname(long addr) { nativeObj = addr; }
 
     public long getNativeObjAddr() { return nativeObj; }
+
+    // internal usage only
+    public static $jname __fromPtr__(long addr) { return new $jname(addr); }