[avb] switch to sdk_source_set()

zx_library() is deprecated, switch the firmware libraries over to
sdk_source_set() instead.

Bug: b/269659863
Change-Id: Ia36691ce1e555e34afb572cc0e6435866652578c

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 2e8d23b..05c49b7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/zircon/zx_library.gni")
+import("//build/cpp/sdk_source_set.gni")
 
 common_vars = {
   sources = [
@@ -65,9 +65,9 @@
 }
 
 # SDK library.
-zx_library("avb") {
-  sdk_publishable = true
-  sdk = "source"
+sdk_source_set("avb") {
+  category = "experimental"
+  sdk_name = "avb"
 
   forward_variables_from(common_vars, "*")