roll: third_party/pigweed pw_thread: Thread priority class

Introduce generic pw::thread::internal::Priority class, which does not
depend on the backend.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/242214
Original-Revision: 4f536c6c1137a7282e7f800aa636c79c5c629191

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 03acee913ad816..4f536c6c1137a7
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8727963216634965569
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia1d64678d597ee521dafe6374cc8e54c70bd2886
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/256314
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 03acee9..4f536c6 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 03acee913ad816b889081af06cad878331a67b62
+Subproject commit 4f536c6c1137a7282e7f800aa636c79c5c629191