roll: third_party/pigweed pw_channel: Wake readers after ForwardingByteChannel writes

- Fix missing Wake() call after after ForwardingByteChannel writes.
- Add multi-step ForwardingByteChannel test to ensure that reading tasks
  are woken.
- Remove unnecessary DoPendWrite mutex locks.

Bugs: b/380295084, b/330788671
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/251912
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: fb156775f2ed50178d899de15491d361f932dfdd

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 52554dee9d6f7f..fb156775f2ed50
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8729543733656750529
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I676563236b256a652627d9f8e7b081f7658db0e5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/251835
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 52554de..fb15677 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 52554dee9d6f7f8fec711f9834b7a55a5440f1ac
+Subproject commit fb156775f2ed50178d899de15491d361f932dfdd