Add defaults support for bootclasspath_fragment module type

This change transforms the bootclsspath_fragment module into a
defaultable module, in order to modify the "art-bootclasspath-fragment"
module definition based on the build flag in
https://r.android.com/2897389

Test: m nothing
Bug: 315027929
Change-Id: I01ba7b15e4fcdea9bd31c369110a3c9ee38bc9f6