Make javamicro a plugin

Treat proto.type: "javamicro" as a plugin by explicitly passing the
path to protoc-gen-javamicro.  This allows removing the javamicro
customizations from the core plugin code.

Bug: 117607748
Test: m checkbuild
Change-Id: I703a8c1b788d77dcf6c656e79cab1c02171aa94e