[ALPS03943817] Add sepolicy for mtk_hal_bluetooth to access nvram

[Detail]
BT vendor lib cannot read the valid BD address from nvram
since selinux violation.
Android P version changes nvdata path to /mnt/vendor/nvdata.
If want to open fstab when using nvram function, it needs
search policy.

[Solution]
allow mtk_hal_bluetooth mnt_vendor_file:dir search

MTK-Commit-Id: 24997cbec894555c537930fe9f10122c450a5e4f

Change-Id: I9279614b100a2675ca88264899125ed549d3301f
CR-Id: ALPS03943817
Feature: BT AOSP