libhidl: introduce Return<void>

Return<void> is a specialization of the generic Return template intended
to be returned from "void" HIDL methods.  It simply encapsulates the
Status object.  With Return<void>, we can auto-generate Return-returning
C++ code all HIDL methods.

b/31367910
b/31348667

Change-Id: If56b4da5117c52e8a1e13bb6d5114ecf576868b4
Signed-off-by: Iliyan Malchev <malchev@google.com>
1 file changed