Add -Wmixed-oneway

Warns if an interface has both oneway and non-oneway methods.

Do not mix oneway with non-oneway methods, because it makes
understanding the threading model complicated for clients and servers.
Specifically, when reading client code of a particular interface, you
need to look up for each method if that method will block or not.

Bug: 168028537
Test: aidl_unittests
Change-Id: If3b4eca0c33532becf7ff90af0d3c6379b7f6821
3 files changed