adb: `features` passes transport features.

`adb features` previously returned a list of host features which was
not terribly useful. This CL changes functionality to return the
transport features instead using the standard targeting args:
  $ adb features  # default target.
  $ adb -e features
  $ adb -s 123456 features

Also adds a "check-feature" service which is currently unused but will
allow the adb client to easily check for a specific feature.

Bug: http://b/23824036
Change-Id: Ibc0c420c75f73d363f3bba7705af616ba2059348