Prevent bootImageVariant.licenseMetadataFile being set twice

Previously, in a build containing source and prebuilt art
bootclasspath_fragments, the bootImageVariant.licenseMetadataFile was
set twice with the source always being set after the prebuilt and
so winning.

This change only sets bootImageVariant.licenseMetadataFile for the
active module so it will use the prebuilt's license file if that is
preferred.

Bug: 245956352
Test: m nothing
Change-Id: I948c7e5123169452f67c85ad98c4bbdb90a5d2de