TBR playground: Move security.Call out of context.T.

This is a companion to the following sequence of changes:
https://vanadium-review.googlesource.com/#/q/topic:ctx_call

Again the purpose is to make our usage of context.T more
consistent, and the JS code is being changed to do the same
thing.

The main change is to get rid of security.GetCall and
security.SetCall.  This means we need to pass security.Call
around explicitly when necessary; rpc.ServerCall includes a
Security() method which returns the security.Call.

Here are the new APIs (none of these included security.Call
before this CL):

Authorize(*context.t, security.Call) error
Grant(*context.T, security.Call) (security.Blessings, error)
Dischargeable(*context.T, security.Call) error
Caveat.Validate(*context.T, security.Call) error
security.RemoteBlessingNames(*context.T, security.Call)
security.LocalBlessingNames(*context.T, security.Call)

MultiPart: 6/7

Change-Id: Iddae58634728eef0e1c3793c4ba7060bcea43ea2
diff --git a/go/src/v.io/x/playground/testdata/src/pong/pong.go b/go/src/v.io/x/playground/testdata/src/pong/pong.go
index 6e171ab..22e780d 100644
--- a/go/src/v.io/x/playground/testdata/src/pong/pong.go
+++ b/go/src/v.io/x/playground/testdata/src/pong/pong.go
@@ -22,8 +22,8 @@
 
 type pongd struct{}
 
-func (f *pongd) Ping(ctx *context.T, _ rpc.ServerCall, message string) (result string, err error) {
-	remote, _ := security.RemoteBlessingNames(ctx)
+func (f *pongd) Ping(ctx *context.T, call rpc.ServerCall, message string) (result string, err error) {
+	remote, _ := security.RemoteBlessingNames(ctx, call.Security())
 	fmt.Printf("%v: %q\n", remote, message)
 	return "PONG", nil
 }