Add SharedMemory API

SharedMemory is parcelable and AIDL friendly
and exposes cool things like dropping write permissions
that are useful for shared memory to have.

Also exposes getFileDescriptor() on MemoryFile
since that was necessary to use MemoryFile
with IPC previously and there are a sizeable
number of reflections that access it, so just make
it public.

Test: SharedMemory CTS tests pass

Change-Id: I71a322abf2eb55267d7299a8e41f847339af8b08