Add LocalVoicemailService API.

Adding support for an OEM LocalVoicemailService which can provide on-device
voicemail capabilities.  Telecom ensures there can only be a single call in
local voicemail processing state.

Flag: com.android.server.telecom.flags.local_voicemail
Test: Added new test app for local testing; CTS to come.
API-Coverage-Bug: b/405460896
Bug: 394367444
Bug: 405460896
Change-Id: I671028a3ba6a8f42fc8e326c7ee4a2da553877e6