bootclasspath_fragment: Add hidden API flag files to snapshot

These are needed at the moment to ensure the hidden API processing
generates the same set of flags whether it is being generated from
source or prebuilts. Soon these will be needed to ensure that the
hidden API flags generated for the individual modules are compatible
with previous releases.

Bug: 179354495
Test: m art-module-sdk and check snapshot zip contains the files
      and the generated Android.bp references them.
Change-Id: I9a3334cc48faa381bbbcbbb59479db719042796a