Give protoc-gen-javastream the ability to output multiple java files.

If there are file-level enums, it still will generate what would have
otherwise been the outer class to hold them, since enums don't get
their own class in the streaming proto scheme.

Make the proto apis not @TestApi for now -- there's too much churn.

Test: make
Change-Id: I01fb7dca935261775efe2573ee009df4d0b07fd0