Make RTPTransceiver Stopped an atomic

Add an accessor to make getting value easy. Also add
TestPeerConnection_SkipStoppedTransceiver. This commit also cleans
up RTPTransceiver creation. We used a helper function, when we should
have just used the provide constructor

atomicbool.go

@@ -12,9 +12,20 @@ func (b *atomicBool) set(value bool) { // nolint: unparam
 		i = 1
 	}
 
-	atomic.StoreInt32(&(b.val), i)
+	atomic.StoreInt32(&b.val, i)
 }
 
 func (b *atomicBool) get() bool {
-	return atomic.LoadInt32(&(b.val)) != 0
+	return atomic.LoadInt32(&b.val) != 0
+}
+
+func (b *atomicBool) compareAndSwap(old, new bool) (swapped bool) {
+	var oldval, newval int32
+	if old {
+		oldval = 1
+	}
+	if new {
+		newval = 1
+	}
+	return atomic.CompareAndSwapInt32(&b.val, oldval, newval)
 }