contexthub: implement findEndpoints in service

- Create callback interface that tracks Endpoint start/stop events
- Maintains an internal cache of the endpoints information, refresh on
  HAL restart.

Test: build
Bug: 375487784
Flag: android.chre.flags.offload_api
Change-Id: I19c4132957b58e626ee7a5c9ba7b45bc13ff67c1