Add kill switch for ViewPool.initPool.

This fixes an issue where, in fast E2E tests, the test can complete before initPool is completed. This results in the ViewPool continuing to create TaskViews and therefore TaskThumbnailViews after RecentsDeps is destroyed. Doing this causes a crash.

Stopping the ViewPool.initPool thread before destroying RecentsDeps fixes this.

Bug: 390156722
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: fast E2E tests in presubmit e.g. AllAppsImageTest
Change-Id: I0f8801ca75cda383df8ed96077cf1b7178771cb6