javaunohelper: Using "final" modifier whenever applicable in java

* com.sun.star.lib.uno.adapter.ByteArrayToXInputStreamAdapter class may be final. There is a overridable method call in the constructor.

* Private methods can't be overridden. The private method "instantiate" is declared final in the com.sun.star.lib.uno.helper.Factory class.

Change-Id: I7dba78a3d1857bbc328f6a581d8e9a5ae130739d
Reviewed-on: https://gerrit.libreoffice.org/11546
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>