Adjust Safety annotation but also just silence lint warning

This external function declaration isn't able to be handled nicely by
clippy in this case, so we suppress the lint. I also added a formatting
fix to make it look more like other Safety sections.

```
error: unsafe function's docs miss `# Safety` section
  --> packages/modules/Bluetooth/system/rust/src/connection/ffi.rs:62:9
   |
62 | /         unsafe fn unchecked_register_rust_callbacks(
63 | |             self: Pin<&mut Self>,
64 | |             callbacks: Box<LeAclManagerCallbackShim>,
65 | |         );
   | |_________^
   |
   = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc
   = note: `-D clippy::missing-safety-doc` implied by `-D warnings`
   = help: to override `-D warnings` add
`#[allow(clippy::missing_safety_doc)]`

error: aborting due to 1 previous error
```

Bug: http://b/346588808
Test: ./toolchain/android_rust/tools/test_compiler.py --prebuilt-path dist/rust-linux.tar.xz --target aosp_cf_x86_64_phone --image --reuse-prebuilt

Bug: 346588808
Test: m aosp_cf_x86_64_phone
Change-Id: I015915f20cd97172bbc66913412da5f139a909de