3/n: Add CoexCoordinator

This change adds CoexCoordinator which intercepts authentication
success/failure signals. The current implementation is pretty much
a no-op and all behavior should still be the same.

Bug: 193089985
Test: Auth on keyguard, BiometricPrompt
Test: Inspect logs, authToken is added when appropriate
Test: atest CoexCoordinatorTest

Change-Id: I53d89e6957da384bcbc2e0fd221d6e5c21b7808f