Make javanano a plugin

Treat proto.type: "javanano" as a plugin by explicitly passing the
path to protoc-gen-javanano.  This allows removing the javanano
customizations from the core plugin code, which no longer supports
javanano natively.

Bug: 117607748
Test: m checkbuild
Change-Id: Id7ed92d45583a7930f35c6734605d9304704bdbc
Merged-In: Id7ed92d45583a7930f35c6734605d9304704bdbc