[8u] RFR(T) 8249610: Make sun.security.krb5.Config.getBooleanObject(String... keys) method public

[Zhengyu Gu]

Please review this trivial patch that makes 
sun.security.krb5.Config.getBooleanObject(String... keys) a public method.

The method was introduced by JDK-8029995 in jdk9, as a public method, 
but made a private method by JDK-8029995 8u backport.

This blocks JDK-8239385 8u backport.

Bug: https://bugs.openjdk.java.net/browse/JDK-8249610
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8249610/webrev.00/

Test:
   jdk_security

Thanks,

-Zhengyu