Warm up Content Capture's BackgroundThread in ActivityThread.

So that BackgroundThread's state will be initiated in the application start up time, instead of the activity start up time. This may reduce the ANR.

Change-Id: I23b99deb03735b2fb6be5d610d27e5f283f609c0
Bug: 408273598
Flag: android.view.contentcapture.flags.warm_up_background_thread_for_content_capture
Test: atest FrameworksCoreTests:android.view.contentcapture.MainContentCaptureSessionTest