[roll third_party/pigweed] pw_rpc: Refactor pw_rpc Java client

- Remove unnecessary, confusing layers of abstraction.
- Simplify working with futures by having the future call class itself
  be a future, rather than provide a future.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/74200

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 793018105f28351..fbec0acd9be7a56
Roller-URL: https://ci.chromium.org/b/8827756876250823761
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8827756876250823761
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3e36f7be1b5e2e4d912c1642b8cf6804a1923167
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/74480
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 7930181..fbec0ac 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 793018105f28351857d8443403fea22ebcb419ae
+Subproject commit fbec0acd9be7a561329635ebf41ab9452f85fa78