contexthub: Make client registration API safer

Modifies the registration API for new endpoint session capabilities to
return a new dedicated IEndpointCommunication interface tied to the
client registration. All endpoint/session APIs have been moved to the
new interface. This prevents security issues resulting from pid re-use.

Adds an unregister() API to IEndpointCommunication for a client to
signal that its associated hub will no longer be active. Subsequent
calls on IEndpointCommunication APIs will fail.

Bug: 384897139
Flag: android.chre.flags.offload_api
Test: Builds
Change-Id: Ibd8a28e96315ca146d62413b5337ece9912d14c4