Export generated AIDL headers

It is common for developers to generate/compile AIDL in a static
library, then link that library into an executable.  When doing this,
developers need to export the generated headers.

Bug: 25779424
Test: a refactoring of the aidl Android.mk shows this works

Change-Id: I4f7d471a601d2a683cb5a9da5e02e3fab576c26a