Choose top-fullscreen main window from task to create task snapshot window.

When creating a task snapshot starting window, the top-most activity
could be a trampoline activity that does not have main window.

Flag: EXEMPT bugfix
Bug: 353187744
Bug: 348733056
Test: verify no regression for b/341020277
Change-Id: I45f1b6570ca5e3e93609188fba14b9d933fbf20a