Support setting file mode

File mode of writable AuthFsEntry can now be changed. The mode is
maintain privately in authfs, but also pass through to fd_server.

Note that this change only aims to support getting/setting the file
mode. The mode is not currently used for ACL check.

Bug: 205169366
Test: atest AuthFsHostTest
Test: atest ComposHostTestCases
Test: composd_cmd forced-odrefresh
      # exit 80 without ART hack, with permissive SELinux
Change-Id: I2405baedae9ba2be5e84eb84d3228f7be080f8c6
diff --git a/authfs/Android.bp b/authfs/Android.bp
index a6792b0..353b597 100644
--- a/authfs/Android.bp
+++ b/authfs/Android.bp
@@ -20,6 +20,7 @@
         "libfuse_rust",
         "liblibc",
         "liblog_rust",
+        "libnix",
         "libstructopt",
         "libthiserror",
     ],