This allows a DRM client to open a locally-cached file on behalf of the
DRM server so the DRM server no longer requires the sdcard_r permission
to access the file's metadata. Specifically, this adds an optional
attribute FileDescriptorKey to the DrmInfoRequest.
This change is dependent on this Widevine CL:
https://widevine-internal-review.googlesource.com/#/c/1330/
Relevant bug reports:
bug: 6426185
Change-Id: Ia7bcb2455c7a55fa4c7c7061de4d672957c7ac0a