microdroid: Replace empty_kernel with :empty_file

Test: TH
Change-Id: I6721c6aaa7f93d2a1e41420cf6ce9c5ef8426ea0
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index de06d01..1092476 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -426,7 +426,7 @@
 
 avb_add_hash_footer {
     name: "microdroid_kernel_signed",
-    src: "empty_kernel",
+    src: ":empty_file",
     filename: "microdroid_kernel",
     partition_name: "boot",
     private_key: ":microdroid_sign_key",
@@ -450,7 +450,7 @@
 
 prebuilt_etc {
     name: "microdroid_kernel",
-    src: "empty_kernel",
+    src: ":empty_file",
     relative_install_path: "fs",
     arch: {
         arm64: {