Use libhidl function to convert to hidl IMemory.

Now that this API is available.

This keeps the implementation details of hidl's
ashmem allocator in libhidl itself (it should be
opaque).

Test: try and succeed watching (part of a) DRM protected movie
Bug: 34234561
Change-Id: Ied9beddace2ccfc859833ac55f4e3837db5f495e