Add support for file descriptors and vectors thereof

We use ScopedFd from nativehelper to manage the lifetime.

Test: Passes AIDL unit and integration tests. Full build completes and
      runs.
Bug: 25242023
Signed-off-by: Casey Dahlin <sadmac@google.com>

Change-Id: I22c87ed1cb3bd4bf667b372ea4a2e9fcd4dd986a