roll: third_party/pigweed pw_thread: Move ThreadCore logic out of pw::Thread

Instead of having pw::Thread convert pw::thread::ThreadCore to a
pw::Function<void()>, define a conversion to pw::Function<void()> on
ThreadCore. This supports existing use cases, while making it possible
to remove ThreadCore from Thread's API.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/253264
Original-Revision: 9e4c976345d4d2de321e17853e9fb86493ff2200

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 019e35f4dba3d1..9e4c976345d4d2
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8728846472191071985
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia576e68cb8929e4d39fd50e46b559244abad7f31
Reviewed-on: https://pigweed-review.googlesource.com/c/gonk/+/253652
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 019e35f..9e4c976 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 019e35f4dba3d103e084903aa3a568f7d67d5143
+Subproject commit 9e4c976345d4d2de321e17853e9fb86493ff2200