[native] Memoize ThreadAncestorsLabel

Summary:
Looking at the profiler flamegraph for the "navigate to thread w/ varun" flow, there are a lot of ThreadAncestorsLabel "slivers" that seem like they might be causing other things to re-render? Unfortunately the profiler isn't super clear so I'm kind of memoizing this on a whim tbh.

Depends on D9212

Test Plan: ThreadAncestorsLabel continues to look as expected.

Reviewers: ginsu, tomek, rohan

Reviewed By: rohan

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9213