pw_rpc: Replace ChannelOutput ID with a name

It is helpful for ChannelOutputs to have a human-readable description
for logging. Replace the ID field, which easily confused with the
channel ID, with a name.

Change-Id: I27a82b303988e718624cb7bbbc2f4bcb5667ac76
diff --git a/pw_rpc/server_test.cc b/pw_rpc/server_test.cc
index 389c9ca..6b5905d 100644
--- a/pw_rpc/server_test.cc
+++ b/pw_rpc/server_test.cc
@@ -61,8 +61,7 @@
       byte(0x82), byte(0x02), byte(0xff), byte(0xff)};
 
   BasicServer()
-      : output_(1),
-        channels_{
+      : channels_{
             Channel::Create<1>(&output_),
             Channel::Create<2>(&output_),
             Channel(),  // available for assignment
@@ -158,7 +157,7 @@
 }
 
 TEST_F(BasicServer, ProcessPacket_UnassignedChannel_AssignsToAvalableSlot) {
-  TestOutput<128> unassigned_output(2);
+  TestOutput<128> unassigned_output;
   server_.ProcessPacket(
       EncodeRequest(PacketType::RPC, /*channel_id=*/99, 42, 27),
       unassigned_output);