Adds an aconfig flag for ranking updates in ashmem

Adds an aconfig flag that moves notification ranking udpates contents,
minus smart actions, into ashmem, for the purposes of preventing binder
transaction failures.

This is already controlled with a Sysprop flag; to enable
teamfooding/dogfooding and future proofing, I'm also adding an aconfig
flag. Either of the flags will enable the feature.

Test: adding flag, successful local build
Bug: 284297289
Change-Id: I4979f0267e95ed33152b03f1ce9d89d7298a26e1