system/rust: Set imports_granularity rustfmt option to Module

This matches the Fuchsia style. See https://fxbug.dev/340943530
for rationale.

I've also reformatted all Rust files under system/rust with this style
as there seemed to be some inconsistencies.

Fix: 396241824
Test: m com.android.btservices
Flag: EXEMPT refactor only
Change-Id: I74425bfbe98a402fa5f9fd5f1746a00b9767a321