Move TestPlatformBootclasspath_Fragments to apex package

This test checks that fragments which are referenced from a
platform_bootclasspath module contribute their hidden API flags to those
used by platform_bootclasspath module. Previously, it was unrealistic
because the bootclasspath_fragment does not belong in an APEX.

This change moves the test from the java package to the apex package to
allow it to be modified to make the bootclasspath_fragment part of an
apex.

Bug: 179354495
Test: m nothing
Change-Id: Icb57f2e1eaea4b14aab5f47f3af7d05ea0555816