sepolicy: Define and allow map permission for vendor dir

This patch tries to provide similar functionality as the previous
change made here:
https://android-review.googlesource.com/#/c/platform/system/sepolicy/+/432339/



Only, making sure we add the same map permissions for the vendor
directory.

Signed-off-by: John Stultz <john.stultz@linaro.org>

(cherry picked from commit 24537b2e)

Bug: 65011018
Test: policy compiles.
Change-Id: I4d0319011ef4ef043134bf299dc4823a6c418717