Improve support for legacy messages

- support legacy `GET_VERSION[_2]` messages
- map errors to error responses
- commonize code layout
- allow non-secure communication on legacy non-secure port

Bug: 197891150
Bug: 258602931
Test: build
Change-Id: Ia87c5e1afd4c33da26047175dde3d5fd045d9d26
1 file changed