bootclasspath_fragment: Require at least one hidden_api package property

Previously, the split_packages, single_packages and package_prefixes
properties were all optional and the split_packages defaulted to ["*"].
As that value conflicted with the other package properties that meant
that split_packages always had to be specified even if it was to just
set it to an empty array.

This change requires at least one of them to be specified and defaults
split_packages to an empty list which means it is not required,
although it can be helpful to make that explicit.

Bug: 194063708
Test: m nothing
Change-Id: I5a4c2d68e72e39f5c4a2441326dfce8685fc8ff2