Block adding toast windows to non-empty tokens. When adding a window of TYPE_TOAST, if addToastWindowRequiresToken is enabled, prevent the addition if the provided WindowToken is not empty. This enforces the constraint that a WindowToken for toasts should only contain a single toast window. Bug: 389950114 Flag: EXEMPT BUGFIX Test: atest WindowManagerServiceTests Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:55aea98481db15689b390c54cae99c409281343f Merged-In: I9f693bb0a28a08d53ad162c1e305e17170185680 Change-Id: I9f693bb0a28a08d53ad162c1e305e17170185680