Add gettid() to SELinux policy in prep for GWP-ASan.

GWP-ASan is a sampled allocator framework that is planned for rollout in
Android R. It instruments the bionic allocator, and requires gettid()
during its initialisation.

Ensure that we can call gettid() from configstore.

Bug: 135634846
Test: N/A

Change-Id: I4a92c989ffde9051831e858e805fc20ae993dc01