Add ability to exclude individual tasks from Recents.

Adds an explicit exclusion control for tasks in the Recents list via a
new `mForceExcludedFromRecents` field in `Task.java`, with accompanying
getter/setter methods. This allows for precise control over task
visibility in Recents, independent of other visibility criteria.

This can replace the previous hidden bubble task approach that used
`isAlwaysOnTop` and `MULTI_WINDOW_MODE` to hide tasks with a more direct
exclusion mechanism.

Bug: 404726350
Flag: com.android.window.flags.exclude_task_from_recents
Test: atest WmTests:RecentTasksTest
Test: atest WmTests:TaskTests
Test: adb shell dumpsys activity recents
Change-Id: I0f8576ba2e9f0095957c440edcf8d5dd639ce657