prebuilt_apis should create a java_system_modules_import

Previously it was creating a java_system_modules which worked fine
because apart from the prebuilt nature the two are functionally
identical. However a follow up change will differentiate between them
which would break this code.

Bug: 182402568
Test: m nothing
Change-Id: Ifc13ce31235494e338d730c61a99d8887c5a2c5b