Fix prebuilt_apis creating of "latest" incompatibilities

Incompatibilities are a bit special in that we only ever want to use
the "latest" if it is for a non-finalized SDK.

Add a new argument, next_api_dir, and use only those files to create
"latest incompatibilities". Create empty incompatibilities for all
modules not in next_api_dir.

Bug: 178171189
Test: m
Change-Id: I727a52e3a45a5e607c6176d481b40fbf495c6eb0