Introduce vendor_microdroid_file for microdroid vendor image

In AVF, virtualizationmanager checks the selinux label of given disk
image for proving whether the given image is edited maliciously.
Existing one(vendor_configs_file, /vendor/etc/*) was too wide to use for this purpose.

Bug: 285854379
Test: m
Change-Id: I6c966c92b238a2262d2eb7f41041ed4c359e9e0a
diff --git a/private/file_contexts b/private/file_contexts
index b7f1a34..32092da 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -412,6 +412,7 @@
 /(vendor|system/vendor)/etc(/.*)?              u:object_r:vendor_configs_file:s0
 /(vendor|system/vendor)/etc/cgroups\.json      u:object_r:vendor_cgroup_desc_file:s0
 /(vendor|system/vendor)/etc/task_profiles\.json    u:object_r:vendor_task_profiles_file:s0
+/(vendor|system/vendor)/etc/avf/microdroid(/.*)?   u:object_r:vendor_microdroid_file:s0
 
 /(vendor|system/vendor)/lib(64)?/egl(/.*)?     u:object_r:same_process_hal_file:s0