apex: set custom_sign_tool

Custom_sign_tool for the Virt apex is set to `sign_virt_apex`. This will
be invoked when the release tool (sign_apex, sign_target_files_apks)
needs to re-sign the APK.

Bug: 193504286
Test: sign_apex --sign_tool sign_virt_apex \
  --payload_key <.pem> --container_key <key> \
  --codename_to_api_level_map Tiramisu:32 \
  -e android.system.virtualmachine.res.apk=PRESIGNED \
  com.android.virt.apex resigned.apex
Change-Id: I30a9367c6a3a4b5036e624ff0bad5abfa899f108
diff --git a/apex/Android.bp b/apex/Android.bp
index 983253e..af65e79 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -13,6 +13,7 @@
 
     key: "com.android.virt.key",
     certificate: ":com.android.virt.certificate",
+    custom_sign_tool: "sign_virt_apex",
 
     // crosvm and virtualizationservice are only enabled for 64-bit targets on device
     arch: {
@@ -100,4 +101,10 @@
             embedded_launcher: true,
         },
     },
+    required: [
+        "img2simg",
+        "lpmake",
+        "lpunpack",
+        "simg2img",
+    ],
 }