Use correct module name for java_import in shouldConvertWithBp2build verification

This change makes sure that prebuilt module has correct module name
prefix("prebuilt_") inside method shouldConvertWithBp2build, to avoid
the bp2build conflict issue when a normal module and a prebuilt module
with the same name are both allowlisted.

Test: CI, added unit tests and the steps to reproduce in b/303376793

Bug: 303376793
Bug: 303868211
Change-Id: I4f1126182ec1f52ae6a08046a77204939230edef