Invalidate task snapshot for protected apps.

Apps in background may have an unprotected recents screenshot available
for launchers that needs to be cleared out when app protection is
applied during screenshare.
Clear the task snapshot in core, also notify recents to invalidate the
original thumbnail in TaskThumbnailCache.

Bug: 329337332
Test: manual
Flag: ACONFIG android.permission.flags.sensitive_content_recents_screenshot_bugfix DEVELOPMENT
Change-Id: Iab342c504fe5e5b76191e9f0aee65c523e6630ca