Avoid outer/inner class name clashes

In some cases, protoc-gen-javastream could emit outer classes
and inner classes with the same name, which is invalid java.

This commit extends protoc-gen-javastream to append an "OuterClass"
suffix, if the outer class name clashes with any message.
This behavior is consistent with the standard protoc.

Bug: 276433199
Test: presubmit
Change-Id: I4e92d70c10c4d92d103b00f15b159acc08dffff2