binder: should use android.os.SharedMemory to transport large messages

[jdcormie]

Is your feature request related to a problem?

Both CPU and wall clock time of sending a message are dominated by the overhead of calling transact(). Today we need O(n) transactions where n is the size of the message.

Describe the solution you'd like

https://developer.android.com/reference/android/os/SharedMemory could be used to send any sized message in O(1) binder transactions.

Describe alternatives you've considered

Apps can already accomplish this using ParcelableMetadata, but it is cumbersome and prevents your app from working with other gRPC transports.