Fix race in C++ AIDL file generation

The export_includes file for a library needs to express a dependency on
all generated exported headers.  For aidl generated headers, express a
dependency on the .cpp file instead, since the generator promises to
generate this file last.  Unfortunately, the C++ headers generated from
a .aidl file depend on the contents of the file.

Change-Id: I9402b364e4538b502c0958ac8c7bd72cb0add724