Convert avbpubkey installation rule to Soong

avbpubkey module type is introduced to convert the installation rule of
system_other.avbpubkey into Soong.

Bug: 376505372
Test: m soong_generated_product_filesystem_test
Change-Id: I66e848ab5f7ca43a38a3edf99111a9da33e3518a
diff --git a/etc/Android.bp b/etc/Android.bp
index 8e043b8..e92437e 100644
--- a/etc/Android.bp
+++ b/etc/Android.bp
@@ -12,6 +12,7 @@
     ],
     srcs: [
         "adb_keys.go",
+        "avbpubkey.go",
         "install_symlink.go",
         "otacerts_zip.go",
         "prebuilt_etc.go",