Allow fastbootd to mount /metadata in recovery.

It is important that fastbootd is able to mount /metadata in recovery, in
order to check whether Virtual A/B snapshots are present. This is
enabled on userdebug builds, but currently fails on user builds.

Fixes:

        audit: type=1400 audit(7258310.023:24): avc:  denied  { mount } for pid=511 comm="fastbootd" name="/" dev="sda15" ino=2 scontext=u:r:fastbootd:s0 tcontext=u:object_r:labeledfs:s0 tclass=filesystem permissive=0

Bug: 181097763
Test: fastboot flash on user build
Change-Id: I1abeeaa3109e08755a1ba44623a46b12d9bfdedc
diff --git a/private/fastbootd.te b/private/fastbootd.te
index 98eb23c..0174faa 100644
--- a/private/fastbootd.te
+++ b/private/fastbootd.te
@@ -38,4 +38,7 @@
 
   # Get fastbootd protocol property
   get_prop(fastbootd, fastbootd_protocol_prop)
+
+  # Mount /metadata to interact with Virtual A/B snapshots.
+  allow fastbootd labeledfs:filesystem { mount unmount };
 ')