libbinder_rs: Add libbinder_rs_on_trusty_mock

Build a mock version of Trusty libbinder_rs
on Android for build testing purposes.

Bug: 242243245
Test: m libbinder_rs_on_trusty_mock
Change-Id: I1b61550af6fbbeac68903c41b5bd20729973b066
diff --git a/libs/binder/rust/sys/lib.rs b/libs/binder/rust/sys/lib.rs
index c5c847b..5352473 100644
--- a/libs/binder/rust/sys/lib.rs
+++ b/libs/binder/rust/sys/lib.rs
@@ -25,7 +25,9 @@
 }
 
 // Trusty puts the full path to the auto-generated file in BINDGEN_INC_FILE
-// and builds it with warnings-as-errors, so we need to use #[allow(bad_style)]
+// and builds it with warnings-as-errors, so we need to use #[allow(bad_style)].
+// We need to use cfg(target_os) instead of cfg(trusty) here because of
+// the difference between the two build systems, which we cannot mock.
 #[cfg(target_os = "trusty")]
 #[allow(bad_style)]
 mod bindings {