aidl: Add end to end test

This test sets up an artificial hierarchy of .aidl files, then
checks that the generated code contains a substring that we
know to be correct.  This isn't a very good unittest, but it does
validate that the whole flow works start to finish.

Bug: 23516947
Test: compiles, unit tests pass.

Change-Id: I931b7659731b05440040623e12596669604cef3a