adb: add client side shell protocol and enable.

Adds the shell protocol functionality to the client side and enables it
if the transport supports the feature.

Bug:http://b/23031026
Change-Id: I9abe1c8b1d39f8dd09666321b1c761ad708a8854
diff --git a/adb/transport.cpp b/adb/transport.cpp
index f6334f7..db9236e 100644
--- a/adb/transport.cpp
+++ b/adb/transport.cpp
@@ -787,7 +787,7 @@
 // The list of features supported by the current system. Will be sent to the
 // other side of the connection in the banner.
 static const FeatureSet gSupportedFeatures = {
-    // None yet.
+        kFeatureShell2,
 };
 
 const FeatureSet& supported_features() {