[roll third_party/pigweed] Rework PW_RPC Android.bp file

Android.bp should declare the current SDK version otherwise the
"private" SDK is used which limits who can call it.

Also, add a target for the echo proto for use by those wanting to create
a demo.

Test: compile Android code
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/78040

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: d7f76fa1614667c..3f126ecfc0651e2
Roller-URL: https://ci.chromium.org/b/8825307748108352849
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8825307748108352849
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I372853ff9c31c3d83d5391a2fb668d342d09b164
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/78162
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 d7f76fa..3f126ec 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d7f76fa1614667c4cacc1cffa17d86b3e3a5394d
+Subproject commit 3f126ecfc0651e2a37a4f0e10b241c615d1057e6