Add callback registration mechanism for listening to communal state changes.

Design: go/communal-manager-api.

Note: this CL also removed "_MANAGER" from the communal service
constant, and "_manager" from the communal service name. When these
became unhidden, ApiLint.kt suggested the changes.
  - http://cs/android/tools/metalava/src/main/java/com/android/tools/metalava/ApiLint.kt;l=2369-2386;rcl=0ef26a465a89c45fcfd24d648883208cd0161fd2

Ignore-AOSP-First: tied to launch of new upcoming hardware.

Test: atest CtsAppTestCases:CommunalManagerTest
Test: atest FrameworksMockingServicesTests:CommunalManagerServiceTest

Bug: 206054365
Change-Id: I6d225d6ddb482d9cffb16d1f9a25a98abd0cc382