[bt-gatt] Move some files for dir structure

Avoid using too many mod.rs (matching style in rest of repo)

Bug: b/308483257
Test: cargo test
Change-Id: Ic6d2184d3a5c506e67e914e886a5ecad7c57c9da
Reviewed-on: https://bluetooth-review.git.corp.google.com/c/bluetooth/+/1341
Reviewed-by: Ani Ramakrishnan <aniramakri@google.com>

rust/bt-gatt/src/central.rs

diff --git a/rust/bt-gatt/src/central/mod.rs b/rust/bt-gatt/src/central.rs
similarity index 100%
rename from rust/bt-gatt/src/central/mod.rs
rename to rust/bt-gatt/src/central.rs

rust/bt-gatt/src/client.rs

diff --git a/rust/bt-gatt/src/client/mod.rs b/rust/bt-gatt/src/client.rs
similarity index 100%
rename from rust/bt-gatt/src/client/mod.rs
rename to rust/bt-gatt/src/client.rs

rust/bt-gatt/src/server.rs

diff --git a/rust/bt-gatt/src/server/mod.rs b/rust/bt-gatt/src/server.rs
similarity index 100%
rename from rust/bt-gatt/src/server/mod.rs
rename to rust/bt-gatt/src/server.rs

rust/bt-gatt/src/types.rs

diff --git a/rust/bt-gatt/src/types/mod.rs b/rust/bt-gatt/src/types.rs
similarity index 100%
rename from rust/bt-gatt/src/types/mod.rs
rename to rust/bt-gatt/src/types.rs