Make javanano a plugin

Treat LOCAL_PROTOC_OPTIMIZE_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: I913d798136fbee9bfaabe1d815b80c85025cb9b9
Merged-In: I913d798136fbee9bfaabe1d815b80c85025cb9b9
diff --git a/core/java_common.mk b/core/java_common.mk
index a23d92d..dfe75f3 100644
--- a/core/java_common.mk
+++ b/core/java_common.mk
@@ -86,6 +86,8 @@
   $(proto_java_srcjar): $(HOST_OUT_EXECUTABLES)/protoc-gen-javamicro
 else ifeq ($(LOCAL_PROTOC_OPTIMIZE_TYPE),nano)
   $(proto_java_srcjar): PRIVATE_PROTO_JAVA_OUTPUT_OPTION := --javanano_out
+  $(proto_java_srcjar): PRIVATE_PROTOC_FLAGS += --plugin=$(HOST_OUT_EXECUTABLES)/protoc-gen-javanano
+  $(proto_java_srcjar): $(HOST_OUT_EXECUTABLES)/protoc-gen-javanano
 else ifeq ($(LOCAL_PROTOC_OPTIMIZE_TYPE),stream)
   $(proto_java_srcjar): PRIVATE_PROTO_JAVA_OUTPUT_OPTION := --javastream_out
   $(proto_java_srcjar): PRIVATE_PROTOC_FLAGS += --plugin=$(HOST_OUT_EXECUTABLES)/protoc-gen-javastream