Fix flaky test at AccessibilityEmbeddedDisplayTest

When starting to track a display, the display's DisplayWindowsObserver
doesn't need to be used and put into sparse array if it is an embedded
display. Then when getting all windows on all displays, this kind of
embedded display won't be accounted into the display lists, and could
fix this flaky issue.

Bug: 142615729
Test: a11y CTS & unit tests
Test: atest AccessibilityEmbeddedDisplayTest -v -- --retry-strategy
 ITERATIONS --max-testcase-run-count 20
Test: Forrest Run 20 times

Change-Id: I1495848ab71162c7a6b19e946c138f4714154679