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.

Change-Id: Ia965df2881cdee8bb5d81278a1eb740def582871
Signed-off-by: John Stultz <john.stultz@linaro.org>