Move ServiceWatcher and dependencies to core

This change moves ServiceWatcher and its dependencies to core and makes
them hidden APIs. This enables system packages like SystemUI to utilize
it for monitoring and stablizing service connections.

Test: able to reference ServiceWatcher from SystemUI
Bug: 375236794
Flag: EXEMPT simply moving files

Change-Id: I4c46836a79dea0456d395b2fde408ad570207cd2