Allow DRM client to pass the FD of an open file to the DRM server.

Part of CL https://googleplex-android-review.googlesource.com/#/c/222797/
This modifies the marshall/unmarshall of IDrmManagerService::
acquireDrmInfo() to watch for DrmInfoRequest tag "FileDescriptorKey".
If tag is present convert string to binary FD, then back to string
after passage through the interface's Binder.

Relevant bug reports:
bug: 6426185

Change-Id: I63748b7c986ca0a89613ed3f1c81f24cffb7a9b2