Flush transactions to ensure new layers are added

A race condition would occur when a new layer was created, but not
yet added by LayerLifecycleManager. Screenshot functions would
attempt to access the created, but unadded layer, resulting an IME
Z-ordering test to create a null buffer in the place of the screenshot
result. Ensure that a transaction flush flag is passed in addClientLayer
so that the LayerLifecycleManager actually adds the layer before it is
accessed.

Fixes: b/424648199
Test: atest ZOrderingTests#testImeScreenshotLayer
Flag: EXEMPT, bug fix
Change-Id: I3d4cc36778f1c31479d0cff98e22e09ee10d7bcb