New, minor ContentCapture APIs for virtual views management.

Test: atest CtsContentCaptureServiceTestCases:android.contentcaptureservice.cts.CustomViewActivityTest
Test: atest CtsContentCaptureServiceTestCases # sanity check
Test: atest FrameworksCoreTests:android.view.contentcapture.ContentCaptureSessionTest
Test: m update-api
Bug: 117944706

Change-Id: I08bed441c6cc15673296f392e333c78018b14f65