Fix race condition causing screenshot view to be added to window twice

In some cases, invoking multiple screenshots in quick succession causes
an error as the decor view is added to the window twice. This change
blocks attempting to attach the window between telling the window
manager to add the decor view and getting the callback that it has
been added.

Bug: 190249438
Fix: 190249438
Test: manual
Change-Id: I2bc3a1322651ab312e75bb9e9426909db368d84e