mt6765-common: sepolicy: Dontaudit untrusted_app to open ashmem_device

* Fixes scroll lag in many apps, such as Twitter:
    W RenderThread: type=1400 audit(0.0:12371): avc: denied { open } for path=/dev/ashmem dev=tmpfs ino=10848 scontext=u:r:untrusted_app:s0:c123,c256,c512,c768 tcontext=u:object_r:ashmem_device:s0 tclass=chr_file permissive=0 app=com.twitter.android
* Apps are no longer allowed open access to /dev/ashmem, unless they
  target API level < Q.
  (https://android.googlesource.com/platform/system/sepolicy/+/8b12ff5f218139102d8eec90c91b114078be4402)

Change-Id: I6405786fea05891642d8437acafcd8c891d75912