Resume root task's children preserving order

Changes #resumeTopActivityUncheckedLocked for the non-leaf case from
resuming top->bottom to resuming bottom->top to ensure the original Z
order is preserved inside desktop windowing root tasks used for "desks".

First, we find the tasks that should be resumed (are visible)
top->bottom to keep the break behavior to avoid unnecesarily resuming
activities that won't be visible in the end.
Then, we resume the tasks found in reverse (bottom->top) order so that
order is kept unchanged.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 414410702
Test: in desktop-first display, open Gmail, then Chrome - verify
"adb shell dumpsys activity activities | grep -e mFocusedApp" shows
Chrome is the focused app, not Gmail

Change-Id: If833078db1ea907ed9e2d5d17b0f309590fca14c