Make ‘rustfmt.toml’ available for other modules

We need the config file to ensure consistent output when running
tests.

Bug: 237977382
Test: manual
(cherry picked from https://android-review.googlesource.com/q/commit:4e86b5e07acd44b04f47e2dcbf3918859d66ccb0)
Merged-In: Ib19d4b739305a5cbed94a40908d76dbbeec67fe5
Change-Id: Ib19d4b739305a5cbed94a40908d76dbbeec67fe5