Decouple rustfmt.toml from soong

With Floss, Flouride/GD is made available outside Android and it's
easier for non-Android development to use rustfmt without soong
dependency.

Bug: 221242765
Test: Call rustfmt on any rust file in packages/modules/Bluetooth

Change-Id: Ic883987500c8129e53e42de3b5250f2126d6f048

rustfmt.toml

-../../../build/soong/scripts/rustfmt.toml
\ No newline at end of file

rustfmt.toml

+# Android Format Style
+
+edition = "2021"
+use_small_heuristics = "Max"
+newline_style = "Unix"