New API: ContentCaptureService.onActivityEvent()

This API is needed for high-level events that happened on activities that are
not whitelisted for Content Capture.

Test: atest CtsContentCaptureServiceTestCases:android.contentcaptureservice.cts.LoginActivityTest#testSimpleLifecycle_defaultSession
Test: atest CtsContentCaptureServiceTestCases # sanity check
Test: m update-api

Bug: 126262658

Change-Id: Id2d4ccfb04d56eba561200d6875374a932c526ae