Don't count toasts as visible windows

in WMS#isAnyWindowVisibleForUid() (now in RootWindowContainer)

Apps could post a toast (or many in a row for that matter) to
trivially bypass the isUidForeground() check.

Bug: 110956953
Test: atest WmTests:ActivityStarterTests
      atest WmTests:RootWindowContainerTests
Test: manual
Change-Id: Ice6571264c35fbf7d4446a5f67a4c44c84d0f9f0