[EXO] Add aidl methods for registering and unregistering a proxy

- Implement register/unregisterProxy minimally. These are mostly
  validity checks. Future CLs will expose the SystemAPI methods on
  A11yManager so an app can call through to these.
- Introduce a ProxyManager skeleton. This will be populated and tested
  in future CLs.

Bug: 241429275
Test: builds, atest AccessibilityManagerServiceTest

Change-Id: Idebf46590b5a129381a34496de67ed18d43369ed