SurfaceView reports setSecure in AssistStructure

Allows Assistant, Contextual Search, Autofill services to identify
regions of the screen where the screenshot will be blank.

Test: atest ContextualSearchManagerTest
Flag: android.app.contextualsearch.flags.report_secure_surfaces_in_assist_structure
Bug: 390504528
Change-Id: Ic8616b6cf7a650a820250a3324abb16f11a53a99