adb: implement shell protocol.

Adds functionality for handling stdin/stdout/stderr streams and exit
codes using the shell protocol.

This CL just contains implementation for adbd which will not yet be
enabled. Once we have the ability to query transport features from the
adb client, another CL will add the implementation for the client side
and update the feature list to turn this on.

Note: this CL must be submitted together with a minadbd CL to update
the service_to_fd() function signature.

Bug: http://b/23030641

Change-Id: Ibed55e9c1946d8a35190696163ff63e8fb880238