Add aconfig flag control to NRU

Adds aconfig flag control to moving Notification Ranking Update
contents to ashmem to prevent binder transaction failures. Removes
sysprop flag.

Test: Unit tests switched to using this flag, run via atest
NotificationRankingUpdateTest.
Bug: 284297289

Change-Id: I5eb966303c3ccda16b396dd61208585dd8ea64d9