proguard rule fixed for [java.lang.ExceptionInInitializerError]

[amrishkakadiya]

with -dontwarn org.apache.poi.** this rule, you may get below error but I have fixed.

java.lang.ExceptionInInitializerError
        at c.a.b.d.b.gb.a(:489)

[flxholle]

In my case this works too:
-keep public class org.apache.poi.hssf.** {*;}
-keep public class org.apache.poi.ss.** {*;}

There's no big difference, but you save at least 500kb of unecessary space in your apk