Reinstate C++ KeyMint impl as default

This is a partial revert of aosp/2856649, changeID
Ia450e9a8f2dc530f79e8d74d7ce65f7d67ea129f, because it turns out
that there are Wear devices that are using the default implementation
(but which are not covered by TreeHugger/presubmit).

It restores the C++ implementation for KeyMint as
android.hardware.security.keymint-service, but preserves the Rust
libraries that were moved here in that CL as a subsequent CL
(aosp/2852598) means that Cuttlefish now uses the libraries here.

Test: VtsAidlKeyMintTargetTest on Cuttlefish
Bug: 328251011
Bug: 314513765
Change-Id: Id6f3491144588e418cd6e18e51262fe839bfdf39