This CL addresses the following denial, when vendor_misc_writer tries to
read DT fstab (i.e. device tree fstab) for /misc entry.

avc: denied { search } for comm="misc_writer" name="android" dev="sysfs" ino=17456 scontext=u:r:vendor_misc_writer:s0 tcontext=u:object_r:sysfs_dt_firmware_android:s0 tclass=dir

DT fstab was used for devices shipped prior to Q, for early-mounting
partitions (e.g. /system, /vendor, /product), which has been disallowed
for Q launch devices. vendor_misc_writer is a new module added since Q,
so it doesn't need to worry about the legacy code path; in practice
there's no benefit of putting /misc entry into DT fstab either.

Bug: 134122603
Bug: 152718206
Test: Build and flash taimen with the change that enables
      vendor_misc_writer. Check that it no longer gives the above denial
      during boot.
Change-Id: Id2fb206706f7cd19a4cde2701e4155bfc03f01b4