Add a new service for AMS structured AIDL support

As a first step to migrate IActivityManager.aidl to structured AIDL,
this CL adds a new service hosting APIs compatible to structured AIDL.

Bug: 419409018
Test: `adb shell dumpsys activity_structured`
Flag: com.android.server.am.enable_activity_manager_structured_service
Change-Id: I15e985d925d188d49339205456bd8e121b003251