Secretkeeper VTS: check for declared instances

Calling binder::get_interface(name) for a name that isn't declared may
take multiple seconds before failing.  Restrict to looking for instance
names that are declared.

Also add a rustfmt.toml file, copied from system/secretkeeper/

Bug: 291228655
Test: VtsSecretkeeperTargetTest on Cuttlefish and Pixel8
Change-Id: I9972bc782fe99f5dc5c2dc3b74fa12a0e000ea6c