This is needed to allow a prebuilt_bootclasspath_fragment to be used
interchangeably with a bootclasspath_fragment in the
platform_bootclasspath module.
The platform_bootclasspath module depends on APEX specific variants of
bootclasspath_fragment modules. That works because the
bootclasspath_fragment modules are part of an apex and so have an APEX
specific variant which the platform_bootclasspath can specify.
Using a prebuilt_bootclasspath_fragment in place of a
bootclasspath_fragment requires that the prebuilt also has an APEX
specific variant.
Specifying exported_bootclasspath_fragments on a prebuilt_apex/apex_set
will cause it to create an APEX variant for the named module whcih will
allow it to be selected by the platform_bootclasspath module.
Bug: 186034565
Bug: 177892522
Test: m nothing
Change-Id: I7ddacc6498ec3a4a9f26c5f78b7f9a033e494d78
023dba0a3f