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
diff --git a/adb/commandline.cpp b/adb/commandline.cpp
index 86f9263..4fe0c25 100644
--- a/adb/commandline.cpp
+++ b/adb/commandline.cpp
@@ -1444,7 +1444,7 @@
         return 0;
     }
     else if (!strcmp(argv[0], "features")) {
-        return adb_query_command("host:features");
+        return adb_query_command(format_host_command("features", transport_type, serial));
     }
 
     usage();