Turn arg_type into a class called AIDLArgument

We also get rid of its linked list property and use std::vector to store lists
of arguments in method_type.

We still expose most of the same data publicly, but there's going to be a lot
of high-touch changes coming with this transition so it's better to break the
change set here.

Change-Id: I71a3dfe1e0fb9cd9437ec81de681b72137fc02cf
Test: unit tests
Bug: 24410295
Signed-off-by: Casey Dahlin <sadmac@google.com>
8 files changed