Make bootclasspath_fragment_sdk_test.go tests more realistic

In the input test fixture this makes sure that every
bootclasspath_fragment is part of an apex and every content module for
the fragment is listed in the appropriate boot jars config property. It
also adds a platform_bootclasspath fragment that references the
fragment.

In the snapshot test fixtures this adds a prebuilt_apex that exports
the fragment so that the platform_bootclasspath has access to the dex
implementation files.

Bug: 177892522
Test: m nothing
Change-Id: I6c73651a359052858232b06229b4433799dd94db