roll: third_party/pigweed: bazel: Provide backend collections as dicts

Implements the pattern suggested in
https://pigweed.dev/bazel_compatibility.html#provide-default-backend-collections-as-dicts.

See http://pwrev.dev/219871 for an example of how this is intended to be
used. We'll probably remove the merge_flags_for_transition_impl and
merge_flags_for_transition_outputs helpers once we've migrated to
platform-based flags.

Original-Bug: 344654805
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/219911
Lint: Ted Pudlik <tpudlik@google.com>

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4a79fe82a35e6b8..db3b2adb2df73dc
Roller-URL: https://ci.chromium.org/b/8743402210436404753
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id466367d8d7c242d37f2d31911f2406d268143ab
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/220116
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.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 4a79fe8..db3b2ad 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4a79fe82a35e6b8e0ad426af19111416c02c7605
+Subproject commit db3b2adb2df73dccb1c42eda709b1b25645d5699