Add TrunkStable flag for task exclude from recents via WCT.

Introduces a new Trunk Stable bug fix flag, `exclude_task_from_recents`,
to enable Window Container Transactions (WCT) to set whether a task
should be excluded from the Recents list. This flag gates the new
`WindowContainerTransaction.setTaskExcludedFromRecents()` API.

Adding this flag is necessary to properly flag the new WCT API.
Attempting to reuse `com.android.wm.shell.enable_create_any_bubble`
resulted in "flag not found" build errors.

Bug: 404726350
Flag: com.android.window.flags.exclude_task_from_recents
Test: m
Change-Id: I9140a8172ca363564e1d4a2b8145f0672a0ee1c4