Add DNS_TETHER to the list of UIDs in the framework.

It has been reserved in android_filesystem_config.h since
aosp/245632 in 2016, but was never included in the list of UIDs
in Process.java.

Bug: 29881876
Test: builds, boots
Change-Id: I6d36d7d15634445d992a10087b5ba31b480f9f20