C++ AIDL on-device GateKeeper / SharedSecret impl

Because Keystore2 always requires valid auth tokens for user keys, this
copies and extends the existing in-guest GateKeeper implementation to
negotiate a shared secret key with Keymint in order to generate a
session key for signing auth tokens.

This implementation also uses AIDL rather than HIDL to interact with the
framework. It's also packaged in an APEX.

The files are cobbled together from a few sources:

- SoftGateKeeper.h is based on hardware/interfaces/gatekeeper/1.0/software/SoftGateKeeper.h
- GateKeeper.{cpp,h} are based on device/google/cuttlefish/guest/hals/gatekeeper/remote/remote_gatekeeper.{cpp,h}
- SharedSecret.{cpp,h} are based on device/google/cuttlefish/guest/hals/keymint/remote/remote_shared_secret.{cpp,h}
- Apex files are based on device/google/cuttlefish/guest/hals/keymint/rust/

Keymint modifications to use BOOT_TIME are lifted from

https://android-review.git.corp.google.com/c/platform/hardware/interfaces/+/2856649/6..8/security/keymint/aidl/default/ta/clock.rs#38

Bug: 332376454
Change-Id: I81845d5e6370bdddb1a24d67437964e03a5fb243
Test: Run with rust-nonsecure keymint on Cuttlefish