backends.rust.apex_available is supported

Apex availability of the rust backend is now correctly applied to the
actual rust lib created.

Bug: 180297657
Test: m com.android.virt
Change-Id: I85a7f48edcb740d54816d4d1c056171368fbc430
diff --git a/build/aidl_interface_backends.go b/build/aidl_interface_backends.go
index d4ae2f8..a100438 100644
--- a/build/aidl_interface_backends.go
+++ b/build/aidl_interface_backends.go
@@ -299,6 +299,7 @@
 		Stem:           proptools.StringPtr("lib" + versionedRustName),
 		Defaults:       []string{"aidl-rust-module-defaults"},
 		Host_supported: i.properties.Host_supported,
+		Apex_available: i.properties.Backend.Rust.Apex_available,
 		Target:         rustTargetProperties{Darwin: perTargetProperties{Enabled: proptools.BoolPtr(false)}},
 	}, &rust.SourceProviderProperties{
 		Source_stem: proptools.StringPtr(versionedRustName),