Prevent windows of embedding activities excluded from a11y

Unable to switch focus to the left pane while a11y
tools enabled because only the top-most visible
window in the Task was interested in A11y.

Allowing the top-most visible window of each TaskFragment
to be focused in a11y.

Bug: 198739177
Test: change focus to left pane in talkback mode
Change-Id: Ie7d7d44018ae4ec7cd17b876f352d43c2f3a11a5