Fix proto flags in java

Compute the common proto flags and pass them to the protoc invocation
when generating java files.

Test: m checkbuild
Change-Id: I0d4c23ad001d01eab03b404545383f009214106d

java/builder.go

@@ -186,7 +186,7 @@ type javaBuilderFlags struct {
 	kotlincFlags     string
 	kotlincClasspath classpath
 
-	protoFlags   string
+	protoFlags   []string
 	protoOutFlag string
 }