Only allow sysui or system to set a Task to always-on-top

Set task always-on-top was introduced in commit 8a168906 and
no permission check was added. Luckily, the attribute only
works if the Task is in multi-window mode.

Bug: 379791774
Test: SafeActivityOptionsTest
Flag: EXEMPT bugfix
Change-Id: I0ca7a3a2e9100c0eae781cac483594dbdf090987