Use Arrays.hashCode in EventIndex hashCode()

The hashCode method on arrays does not actually hash the array contents
but instead is just the identity of the array itself. This will return
erroneous results for objects that are equal but not the exact same
instance.

Bug: 217923092
Test: errorprone build
Change-Id: Idfcc220da5fb07a9b24a7b12795466303b14e35a