Make bootclasspath_fragments always perform hidden API processing

Previously, bootclasspath_fragment modules would only perform hidden
API processing if they provided some stub libraries and fragments. That
was needed because the bootclasspath_fragment modules were added before
Soong supported hidden API processing on all the different modules and
before they all provided the necessary information that hidden API
processing required.

This change stops hidden API being conditional as it is no longer
required as it has been enabled on all existing bootclasspath_fragment
modules.

Bug: 179354495
Test: m nothing
Change-Id: I0cbf11986adff1f2f967b96f86e6bfe0e9b8b1ef